RNA vaccine The present invention relates to RNA vaccines, and particularly, although not exclusively, to novel uses and methods for thermally stabilising RNA vaccine formulations, and especially the thermal stabilisation of self-amplifying RNA vaccine formulations. The invention extends to novel vaccine compositions and formulations of temperature stabilised RNA vaccines, and their use in therapy, for example in treating and preventing disease, such as a bacterial or viral infection, and/or in vaccine delivery. The invention also extends to vaccine vials and pre-loaded syringes comprising the novel, thermally stabilised RNA vaccine formulations. The severe acute respiratory syndrome coronavirus (SARS-CoV-2), causing 2019-nCoV or COVID-19 disease, is a virus belonging in the coronavirus (CoV) group of disease- causing pathogens that includes severe acute respiratory syndrome coronavirus (SARS) and Middle East respiratory syndrome-related coronavirus (MERS). Coronaviruses are usually restricted to their wild hosts. However, both SARS and MERS, and more recently SARS-CoV-2, have all been transferred to humans, and this caused the SARS and MERS outbreaks of 2003, 2012 and 2019, respectively. During 2020, there has been an urgent global initiative to generate an effective vaccine for immunising the world’s population against SARS-CoV-2. Various different approaches are being used to create an effective vaccine, including inactivated attenuated virus (e.g. Sinovac Biotech Ltd.; and Valneva SE), adenovirus DNA vector (e.g. Oxford University & AstraZeneca plc; and Janssen Pharmaceuticals), and messenger RNA (Pfizer Inc. & BioNTech; and Moderna). However, a significant problem with each of these vaccines is that their manufacture requires production facilities which are able to process thousands of litres of material in order to produce a sufficient number of doses. This is because these vaccines require a relatively large dose of the DNA or mRNA material for both the primer and booster jabs. More recently, therefore, self-amplifying (or self-replicating) naked RNA vectors (saRNA replicons) have come into focus as a promising new vaccine technology platform. Self-amplifying RNA replicons address many of the problems associated with conventional DNA and mRNA vaccines that have struggled to demonstrate their full potential. Using saRNA offers significant advantages over mRNA or DNA vaccine candidates as they can yield exponentially higher levels of protein expression, and so much lower doses are required to stimulate an immune response. As shown in Figure 1, saRNA vaccines are synthesised synthetically without the complications of a packaging cell line, contamination with replication-competent virus and anti-vector immunity. Once synthesised, the saRNA can be suitably formulated, for example, within lipid nanoparticles, and then injected into muscle as with a conventional vaccine. The saRNA is delivered into the cytoplasm of muscle cells, whereupon the RNA self-amplifies itself in the muscle cells and expresses the vaccine antigen. In the case of SARS-CoV-2, the causative agent of COVID-19, the antigen is the viral spike glycoprotein. The immune system senses the saRNA-encoded spike glycoprotein in the same way as in an active infection, and this “mock” infection induces the production of protective antibodies and T-cells making the vaccinated individual immune to the infectious virus and protecting them for subsequent infections. As is illustrated in Figure 2, the inventors have identified the synthetic self-amplifying RNA platform as the most cost-effective approach to develop their COVID-19 vaccine. Their chosen vector is based on a non-infectious Venezuelan Equine Encephalitis Virus (VEEV) replicon backbone encoding non-structural proteins required for the self- amplification step, where the CoV-2 S glycoprotein has been inserted in place of structural genes downstream of the subgenomic promoter (SGP). SaRNA is strictly confined to the cytosol, does not require a cDNA intermediary or penetration into the nucleus, and can generate very high expression of a gene product with a surprisingly low initial dose (i.e. ≤1µg to around 5µg saRNA/subject). Formulated saRNA is taken up into the cytoplasm of target cells, which leads to intracellular amplification of the saRNA by the encoded polymerase machinery and very high expression levels of the viral antigen. In contrast, however, traditional DNA and mRNA vaccines have no replicative capacity and therefore require much higher doses (>100 fold compared to saRNA) of material to elicit similar immune responses. Indeed, the mRNA approaches used by Moderna and Pfizer/BioNTech require a vaccine dose of at least 300-400µg of non-amplifying mRNA compared to the 1µg-5µg saRNA of the inventors’ self-amplifying vaccine. Accordingly, these approaches typically require at least 300-400 fold more RNA product to generate the same level of immune response as saRNA, and, as a result, they will struggle to scale-up sufficient quantities of their mRNA vaccine supplies. The requirement for higher doses of mRNA vaccine is also associated with greater side effects, both in terms of their frequency and magnitude. Given that a saRNA vaccine can be administered at significantly lower doses compared to an mRNA vaccine, the vials containing saRNA vaccine doses can be produced with much lower nucleic acid concentrations than for the corresponding mRNA vaccines. For example, a small dose saRNA vaccine vial containing 1-5 doses would contain only 1-25 µg nucleic acid, whereas the equivalent small dose mRNA vaccine vial for 1-5 doses would contain at least 300-1500 µg nucleic acid. Hence, the concentration of nucleic acid contained in lose dose saRNA vaccine vials is significantly lower than for a corresponding mRNA vaccine vial. Most vaccines, particularly live vaccines, require the “cold chain”, as vaccine efficacy can be significantly reduced if they are not stored in a temperature range of 2-8°C at all times, i.e. from vaccine production to dispensation to patients. Any failure to maintain the cold chain can result in wastage or administration of ineffective vaccines. This necessity places a tremendous financial and logistical burden on vaccination programs, particularly in the developing world. Indeed, another major problem suffered by all currently available RNA vaccines, be they either mRNA or saRNA, is that, due to the inherent instability of RNA, it is an essential requirement for them to be stored at very cold temperatures, and far lower than 2-8°C. For example, Moderna’s mRNA vaccine must be stored at -20°C, whereas Pfizer/BioNTech’s mRNA vaccine has to be maintained at -70°C. Clearly, this raises a further significant logistical problem when rolling out a mass immunisation campaign with RNA vaccines, because of the need for specialised freezers at the site of where the immunisations are being conducted. Accordingly, there is a need in the art to provide new methods for thermally stabilising vaccines, especially RNA vaccines, which contain low concentrations of the nucleic acid, such as mRNA or saRNA, so that they can be stored at higher temperatures (ideally above -20°C), thereby avoiding the need to use specialised freezers. The inventors have therefore investigated ways in which low concentrations of their saRNA vaccine (e.g.2µg/ml RNA) formulated in lipid (e.g. encapsulated lipid-based nanoparticle) can be thermally stabilised for extended periods of time (i.e. at least five months). They have surprisingly found that the use of 0.5% (w/v) and 1% (w/v) carbohydrate (and in particular, the disaccharide trehalose) prolongs the thermal stability of lipid-formulated RNA when in low concentrations at temperatures as high as about 2-8°C for at least five months. Accordingly, in a first aspect of the invention, there is provided the use of a carbohydrate or a pharmaceutically acceptable complex, salt, solvate, tautomeric form, stereoisomer or polymorphic form thereof, to thermally stabilise an RNA vaccine formulation comprising a lipid and an RNA construct encoding an antigen of interest, wherein the RNA construct is at a concentration of less than about 200µg/ml. In a second aspect of the invention, there is provided an RNA vaccine formulation comprising a lipid, an RNA construct encoding an antigen of interest, and a carbohydrate or a pharmaceutically acceptable complex, salt, solvate, tautomeric form, stereoisomer or polymorphic form thereof, wherein the weight ratio of RNA construct to carbohydrate or a pharmaceutically acceptable complex, salt, solvate, tautomeric form, stereoisomer or polymorphic form thereof is between 1:5 and 1:500,000. In a third aspect, there is provided a method of preparing an RNA vaccine formulation comprising a lipid, an RNA construct encoding an antigen of interest, and a carbohydrate or a pharmaceutically acceptable complex, salt, solvate, tautomeric form, stereoisomer or polymorphic form thereof, wherein the method comprises contacting the RNA construct and the carbohydrate or a pharmaceutically acceptable complex, salt, solvate, tautomeric form, stereoisomer or polymorphic form thereof at a weight ratio of RNA construct to carbohydrate or a pharmaceutically acceptable complex, salt, solvate, tautomeric form, stereoisomer or polymorphic form thereof is between 1:5 and 1:500,000. In a fourth aspect, there is provided a vaccine vial or a pre-loaded syringe comprising the RNA vaccine formulation of the second aspect. In a fifth aspect, there is provided an RNA vaccine formulation according to the second aspect, for use in therapy. In a sixth aspect, there is provided an RNA vaccine formulation according to the second aspect, for use in eliciting an immune response. In a seventh aspect, there is provided a method of eliciting an immune response in a subject, the method comprising administering, to a subject in need thereof, a therapeutically effective amount of an RNA vaccine formulation according to the second aspect. Advantageously, as described in the Examples, the inventors have surprisingly demonstrated that a lipid-based vaccine formulation comprising low concentrations of RNA (e.g.2µg/ml RNA) is thermally stabilised by 0.5% or 1.0% (w/v) trehalose, i.e. a weight ratio of RNA:trehalose of 1:2,500 or 1:5,000, respectively. The ability to thermally stabilise a vaccine formulation having such low concentrations of RNA with such low concentrations of a carbohydrate, such as trehalose, is particularly advantageous when preparing low dose vaccine vials, such as the vial or syringe according to the fourth aspect, for logistical reasons. In addition, as described in Example 2, the inventors have shown that the thermal protective effects of the carbohydrate (e.g. trehalose) surprisingly require an initial freeze shock treatment (i.e. freeze-thaw) followed by addition of the carbohydrate excipient post thaw. The surprising discovery that a freeze-thaw treatment is required for the carbohydrate to provide product stabilising activity was totally unexpected. Although they do not wish to be bound by any hypothesis, the inventors believe that the freezing-thawing mechanism causes a change in the product that allows carbohydrate to interact with the product (e.g. LNP formulated saRNA) in a manner that is different to the product that has not been subjected to a freeze shock step. Preferably, the RNA vaccine formulation of the second aspect is thermally stabilised. It will be appreciated that the expression “thermal stabilisation” can mean that the vaccine formulation substantially retains its biological activity (i.e. it elicits an immune response in a subject administered with the construct) when stored at certain temperatures for a period of time. Although the inventors do not wish to be bound by any hypothesis, they believe that the thermal stabilisation effects may be realised by stabilising the lipid in the formulation, for example by preventing or its reducing aggregation, and/or by stabilising the RNA per se in the lipid. Whether or not an immune response is elicited, or the extent thereof, can be determined by detecting the presence of immunospecific antibodies (e.g., IgG) raised against the antigen of interest encoded by the RNA construct. For example, the antigen of interest may be a viral coat protein, such as the spike protein for SARS-CoV-2. Preferably, the vaccine formulation is thermally stabilised following storage at a temperature of -20°C and above, more preferably -15°C and above, and most preferably -10°C and above. Preferably, the vaccine formulation is thermally stabilised following storage at a temperature of -5°C and above, more preferably 0°C and above, and most preferably 1°C and above. Most preferably, the vaccine formulation is thermally stabilised following storage at a temperature of 2°C and above, more preferably 3°C and above, and most preferably 4°C and above. Even more preferably, the vaccine formulation is thermally stabilised following storage at a temperature of 5°C and above, more preferably 6°C and above, and most preferably 7°C and above. Preferably, the vaccine formulation is thermally stabilised following storage at a temperature of less than 40°C, more preferably less than 35°C, and most preferably less than 30°C. Preferably, the vaccine formulation is thermally stabilised following storage at a temperature of less than 25°C, more preferably less than 20°C, and most preferably less than 15°C. Preferably, the vaccine formulation is thermally stabilised following storage at a temperature of less than 10°C, more preferably less than 8°C, and most preferably less than 7°C. Preferably, the vaccine formulation is thermally stabilised following storage at a temperature of between -20°C and 40°C, more preferably between -15°C and 25°C, and most preferably between -10°C and 20°C. Preferably, the vaccine formulation is thermally stabilised following storage at a temperature of between -5°C and 15°C, more preferably between 0°C and 10°C, preferably between 1°C and 9°C, and most preferably between 2°C and 8°C. Preferably, the vaccine formulation is thermally stabilised following storage for at least 1, 2, 3, 4, 5, 6 or 7 days. Preferably, the vaccine formulation is thermally stabilised following storage for at least 1, 2, 3, 4, 5, 6 or 7 weeks. Preferably, the vaccine formulation is thermally stabilised following storage for at least 1, 2, 3, 4, 5, 6 or 7 months. More preferably, the vaccine formulation is thermally stabilised following storage for at least 8, 9, 10, 11 or 12 months. It will be appreciated that the invention relates to the use or presence of a carbohydrate, or a pharmaceutically acceptable complex, salt, solvate, tautomeric form, stereoisomer or polymorphic form thereof, each of which are implicitly covered herein. As such, when any carbohydrate is described herein, it is envisaged that it also relates to a pharmaceutically acceptable complex, salt, solvate, tautomeric form, stereoisomer or polymorphic form of that carbohydrate. The carbohydrate may be referred to as a sugar. The carbohydrate may be a monosaccharide, which may be selected from a group consisting of: glucose; galactose; fructose; and xylose. The carbohydrate may be a disaccharide, which may be selected from a group consisting of: sucrose; lactose; maltose; isomaltose; and trehalose. The carbohydrate may be a polyol, which may be selected from a group consisting of: sorbitol; and mannitol and glycerol. As described in the Examples, however, it is preferred that the carbohydrate is a disaccharide, and most preferably trehalose, or a pharmaceutically acceptable complex, salt, solvate, tautomeric form, stereoisomer or polymorphic form thereof. The trehalose may be synthetic trehalose. A pharmaceutically acceptable “complex” of the carbohydrate may be understood to be a multi-component complex, wherein the carbohydrate and at least one other component are present in stoichiometric or non-stoichiometric amounts. The complex may be other than a salt or solvate. Complexes of this type include clathrates (carbohydrate-host inclusion complexes) and co-crystals. The latter are typically defined as crystalline complexes of neutral molecular constituents which are bound together through non-covalent interactions, but could also be a complex of a neutral molecule with a salt. Co-crystals may be prepared by melt crystallisation, by recrystallisation from solvents, or by physically grinding the components together. The term “pharmaceutically acceptable salt” may be understood to refer to any salt of a carbohydrate provided herein which retains its biological properties and which is not toxic or otherwise undesirable for pharmaceutical use. Such salts may be derived from a variety of organic and inorganic counter-ions well known in the art. Such salts include, but are not limited to: (1) acid addition salts formed with organic or inorganic acids such as hydrochloric, hydrobromic, sulfuric, nitric, phosphoric, sulfamic, acetic, adepic, aspartic, trifluoroacetic, trichloroacetic, propionic, hexanoic, cyclopentylpropionic, glycolic, glutaric, pyruvic, lactic, malonic, succinic, sorbic, ascorbic, malic, maleic, fumaric, tartaric, citric, benzoic, 3-(4-hydroxybenzoyl)benzoic, picric, cinnamic, mandelic, phthalic, lauric, methanesulfonic, ethanesulfonic, 1,2-ethane-disulfonic, 2- hydroxyethanesulfonic, benzenesulfonic, 4-chlorobenzenesulfonic, 2- naphthalenesulfonic, 4-toluenesulfonic, camphoric, camphorsulfonic, 4- methylbicyclo[2.2.2]-oct-2-ene-1-carboxylic, glucoheptonic, 3-phenylpropionic, trimethylacetic, tert-butylacetic, lauryl sulfuric, gluconic, benzoic, glutamic, hydroxynaphthoic, salicylic, stearic, cyclohexylsulfamic, quinic, muconic acid and the like acids; or (2) base addition salts formed when an acidic proton present in the parent compound either (a) is replaced by a metal ion, e.g., an alkali metal ion, an alkaline earth ion or an aluminium ion, or alkali metal or alkaline earth metal hydroxides, such as sodium, potassium, calcium, magnesium, aluminium, lithium, zinc, and barium hydroxide, ammonia or (b) coordinates with an organic base, such as aliphatic, alicyclic, or aromatic organic amines, such as ammonia, methylamine, dimethylamine, diethylamine, picoline, ethanolamine, diethanolamine, triethanolamine, ethylenediamine, lysine, arginine, ornithine, choline, N,N′-dibenzylethylene-diamine, chloroprocaine, diethanolamine, procaine, N-benzylphenethylamine, N- methylglucamine piperazine, tris(hydroxymethyl)-aminomethane, tetramethylammonium hydroxide, and the like. Pharmaceutically acceptable salts may include, sodium, potassium, calcium, magnesium, ammonium, tetraalkylammonium and the like, and when the compound contains a basic functionality, salts of non-toxic organic or inorganic acids, such as hydrohalides, e.g. hydrochloride, hydrobromide and hydroiodide, carbonate or bicarbonate, sulfate or bisulfate, borate, phosphate, hydrogen phosphate, dihydrogen phosphate, pyroglutamate, saccharate, stearate, sulfamate, nitrate, orotate, oxalate, palmitate, pamoate, acetate, trifluoroacetate, trichloroacetate, propionate, hexanoate, cyclopentylpropionate, glycolate, glutarate, pyruvate, lactate, malonate, succinate, tannate, tartrate, tosylate, sorbate, ascorbate, malate, maleate, fumarate, tartarate, camsylate, citrate, cyclamate, benzoate, isethionate, esylate, formate, 3-(4- hydroxybenzoyl)benzoate, picrate, cinnamate, mandelate, phthalate, laurate, methanesulfonate (mesylate), methylsulphate, naphthylate, 2-napsylate, nicotinate, ethanesulfonate, 1,2-ethane-disulfonate, 2-hydroxyethanesulfonate, benzenesulfonate (besylate), 4-chlorobenzenesulfonate, 2-naphthalenesulfonate, 4-toluenesulfonate, camphorate, camphorsulfonate, 4-methylbicyclo[2.2.2]-oct-2-ene-1-carboxylate, glucoheptonate, 3-phenylpropionate, trimethylacetate, tert-butylacetate, lauryl sulfate, gluceptate, gluconate, glucoronate, hexafluorophosphate, hibenzate, benzoate, glutamate, hydroxynaphthoate, salicylate, stearate, cyclohexylsulfamate, quinate, muconate, xinofoate and the like. Hemisalts of acids and bases may also be formed, for example, hemisulphate salts. The skilled person will appreciate that the aforementioned salts include ones wherein the counterion is optically active, for example D-lactate, or racemic, for example DL- tartrate. The term “solvate” may be understood to refer to a carbohydrate provided herein or a salt thereof, that further includes a stoichiometric or non-stoichiometric amount of solvent bound by non-covalent intermolecular forces. Where the solvent is water, the solvate is a hydrate. Pharmaceutically acceptable solvates in accordance with the invention include those wherein the solvent of crystallization may be isotopically substituted, e.g. D ₂ O, d ₆ -acetone and d ₆ -DMSO. In one embodiment, the concentration of carbohydrate in the vaccine formulation may be at least 0.01% (w/v), 0.05% (w/v) or 0.10% (w/v). The concentration of carbohydrate in the vaccine formulation may be at least 0.15% (w/v), 0.20% (w/v) or 0.25% (w/v). The concentration of carbohydrate in the vaccine formulation may be at least 0.30% (w/v), 0.35% (w/v) or 0.40% (w/v). The concentration of carbohydrate in the vaccine formulation may be at least 0.45% (w/v), 0.50% (w/v) or 0.55% (w/v). A preferred concentration of the carbohydrate in the vaccine formulation is about 0.5% (w/v). In another embodiment, the concentration of carbohydrate in the vaccine formulation may be at least 0.55% (w/v), 0.60% (w/v) or 0.65% (w/v). The concentration of carbohydrate in the vaccine formulation may be at least 0.70% (w/v), 0.75% (w/v) or 0.80% (w/v). The concentration of carbohydrate in the vaccine formulation may be at least 0.85% (w/v), 0.90% (w/v) or 0.95% (w/v). A preferred concentration of the carbohydrate in the vaccine formulation is about 1.0% (w/v). In a further embodiment, the concentration of carbohydrate in the vaccine formulation may be at least 2% (w/v), 3% (w/v), 4% (w/v), or 5% (w/v). More preferably, the concentration of carbohydrate in the vaccine formulation may be at least 6% (w/v), 7% (w/v), 8% (w/v), 9% (w/v), or 10% (w/v).The concentration of carbohydrate in the vaccine formulation may be at least 11% (w/v), 12% (w/v), 13% (w/v), 14% (w/v), or 15% (w/v). The concentration of carbohydrate in the vaccine formulation may be at least 16% (w/v), 17% (w/v), 18% (w/v), 19% (w/v), or 20% (w/v). Suitably, the concentration of carbohydrate in the vaccine formulation is less than 30% (w/v), 25% (w/v), 20% (w/v), 15% (w/v), or 10% (w/v). More suitably, the concentration of carbohydrate in the vaccine formulation is less than 9% (w/v), 8% (w/v), 7% (w/v), 6% (w/v), or 5% (w/v). Most suitably, the concentration of carbohydrate in the vaccine formulation is less than 4% (w/v), 3% (w/v), 2% (w/v), 1.5% (w/v), or 1.0% (w/v). In one embodiment, the concentration of carbohydrate in the vaccine formulation may be between 0.01 and 50% (w/v), between 0.05 and 40% (w/v) or between 0.10 and 30% (w/v). The concentration of carbohydrate in the vaccine formulation may be between 0.15 and 25% (w/v), between 0.20 and 20% (w/v) or between 0.25 and 15% (w/v). The concentration of carbohydrate in the vaccine formulation may be between 0.30 and 10% (w/v), between 0.35 and 7% (w/v) or between 0.40 and 5% (w/v). The concentration of carbohydrate in the vaccine formulation may be between 0.45 and 3% (w/v), between 0.50 and 2% (w/v) or between 0.55 and 1.5% (w/v). A preferred concentration of the carbohydrate in the vaccine formulation is between 0.5 and 1% (w/v). Preferably, the RNA construct in the vaccine formulation according to the invention is at a concentration of less than about 200µg/ml. Suitably, the RNA construct is at a concentration of less than about 190µg/ml, 180µg/ml, 170µg/ml or 160µg/ml. More suitably, the RNA construct is at a concentration of less than about 150µg/ml, 140µg/ml, 130µg/ml or 120µg/ml. Still more suitably, the RNA construct is at a concentration of less than about 110µg/ml, 100µg/ml, 90µg/ml or 80µg/ml. Preferably, the RNA construct is at a concentration of less than about 70µg/ml, 60µg/ml, 50µg/ml or 40µg/ml. More preferably, the RNA construct is at a concentration of less than about 30µg/ml, 25µg/ml, 24µg/ml, or 23µg/ml. Even more preferably, the RNA construct is at a concentration of less than about 22µg/ml, 21µg/ml, or 20µg/ml. Still more preferably, the RNA construct is at a concentration of less than about 19µg/ml, 18µg/ml, 17µg/ml or 16µg/ml. Still more preferably, the RNA construct is at a concentration of less than about 15µg/ml, 14µg/ml, 13µg/ml, 12µg/ml, or 11µg/ml. Still more preferably, the RNA construct is at a concentration of less than about 10µg/ml, 9µg/ml, 8µg/ml, 7µg/ml, 6µg/ml or 5µg/ml. Even more preferably, the RNA construct is at a concentration of less than about 4µg/ml, 3µg/ml, 2.5µg/ml or 2.2µg/ml. Preferably, the RNA construct is at a concentration of more than about 0.01µg/ml, 0.05µg/ml, 0.075µg/ml or 0.1µg/ml. More preferably, the RNA construct is at a concentration of more than about 0.015µg/ml or 0.017µg/ml. A preferred concentration of the RNA construct is about 0.2µg/ml. Preferably, the weight ratio of RNA construct to carbohydrate in the vaccine formulation is between 1:10 and 1:250,000, between 1:50 and 1:100,000, between 1:100 and 1:50,000, between 1:250 and 1:25,000, between 1:500 and 1:50,000 or between 1:750 and 1:20,000, more preferably between 1:1,000 and 1:10,000 or between 1:1,500 and 1:7,500, most preferably between 1:2,000 and 1:6,000. In some embodiments, the weight ratio of RNA construct to carbohydrate in the vaccine formulation is between 1:2,000 and 1:5,000, between 1:2,100 and 1:4,000, between 1:2,200 and 1:3,000, between 1:2,300 and 1:2,750 or between 1:2,400 and 1:2,600. In some embodiments, the weight ratio of RNA construct to carbohydrate in the vaccine formulation is about 1:2,500. In some embodiments, the weight ratio of RNA construct to carbohydrate in the vaccine formulation is between 1:3,000 and 1:6,000, between 1:4,000 and 1:5,800, between 1:4,500 and 1:5,500, between 1:4,750 and 1:5,250 or between 1:4,900 and 1:5,100. In some embodiments, the weight ratio of RNA construct to carbohydrate in the vaccine formulation is about 1:5,000. The RNA construct may be encapsulated in a carrier particle comprising lipid. Although not wishing to bound by hypothesis, the inventors believe that the vaccine formulation may be stabilised due to the carbohydrate interacting with polar head groups of the lipid in the carrier particle replacing the water in between these head groups, and thereby reducing lipid water contact and potential for RNA hydrolysis. The carried particle may be a nanoparticle which comprises lipid, i.e. the nanoparticle is lipid- based. For example, this may be the situation in which the RNA construct is formulated in a lipid-based nanoparticle or Lipid Nano Particle (LNP). The nanoparticle may comprise one or more components selected from a group consisting of: a cationic lipid (which is preferably ionisable); phosphatidylcholine; cholesterol; and polyethylene glycol (PEG)-lipid. A preferred nanoparticle comprises an ionisable cationic lipid, phosphatidylcholine, cholesterol, and polyethylene glycol (PEG)-lipid. The cationic lipid may be D-Lin-MC3-DMA. D-Lin-MC3-DMA is also known as (6Z,9Z,28Z,31Z)-Heptatriaconta-6,9,28,31-tetraen-19-yl 4-(dimethylamino)butanoate. Thus, another preferred nanoparticle comprises an ionisable cationic lipid (preferably D-Lin-MC3-DMA), phosphatidylcholine, cholesterol, and polyethylene glycol (PEG)- lipid. Preferably, the phosphatidylcholine is distearoylphosphatidylcholine (DSPC). Preferably, the PEG is 1,2-Dimyristoyl-sn-Glycero-3-Phosphoethanolamine (DMPE) DMPE-PEG. The average diameter of the nanoparticle may be between 30nm and 500nm, or between 40 and 200nm, and preferably between 50 and 150nm. In another embodiment, the RNA construct is formulated within a liposome. Liposomes are known to the skilled person as being a sac of phospholipid molecules which encapsulate an active agent, i.e. the RNA construct. The average diameter of the liposome may be between 30 and 3000nm, and preferably between 40 and 2000nm, more preferably between 50 and 1000nm. In yet another embodiment, the RNA construct is formulated within a polyplex. The vaccine formulation may comprise an adjuvant. In some embodiments, the vaccine formulation may not comprise an adjuvant. For example, this may be the situation in which the RNA is formulated in an LNP. The method of the third aspect results in a surprisingly thermostable RNA vaccine solution comprising lipid, an RNA construct and a carbohydrate. The method may comprise preparing the RNA construct in an aqueous solution prior to mixing with lipid components. The lipid components may be dissolved in ethanol. The lipid components may comprise cholesterol, phosphatidylcholine (e.g. DSPC), cationic lipid and/or PEGylated lipid. The RNA solution may then be mixed with the lipid solution to result in the formation of nanoparticles in which the RNA construct is encapsulated by a lipid layer. Once the nanoparticles containing the RNA construct have formed, the ethanol may be removed, for example by Tangential Flow Filtration. As described in Example 2, the inventors were surprised to observe that the thermal protective effects of the carbohydrate (e.g. trehalose) require an initial freeze shock treatment of the RNA and/or lipid (e.g. the RNA/lipid nanoparticles), followed by thawing (i.e. freeze-thaw), which is then followed by contacting the RNA/lipid nanoparticles with the carbohydrate excipient. The surprising finding that a freeze- thaw treatment is required for the carbohydrate to provide stabilising activity was totally unexpected. Thus, the method of the third aspect preferably comprises freezing the RNA construct and/or lipid (or RNA construct/lipid nanoparticles) prior to contact with the carbohydrate. The method of the third aspect preferably comprises thawing the frozen RNA construct and/or lipid (or RNA construct/lipid nanoparticles) prior to contact with the carbohydrate. Similarly, preferably the use of the first aspect or the RNA vaccine formulation of the second aspect comprises subjecting the RNA vaccine formulation to a freeze-thaw treatment prior to contacting the RNA construct and/or lipid with the carbohydrate. The freeze-thaw treatment preferably comprises: (i) freezing the RNA construct and/or lipid; (ii) subsequently thawing the frozen RNA construct and/or lipid; and then (iii) subsequently contacting the thawed RNA construct and/or lipid with the carbohydrate. Preferably, the RNA construct and/or lipid is exposed to a temperature of 0°C or lower, -5°C or lower, or -10°C or lower. Preferably, the RNA construct and/or lipid is exposed to a temperature of -15°C or lower, -20°C or lower, or -25°C or lower. Preferably, the RNA construct and/or lipid is exposed to a temperature of -30°C or lower, -35°C or lower, or -40°C or lower. Preferably, the RNA construct and/or lipid is exposed to a temperature of -45°C or lower, -50°C or lower, or -55°C or lower. Preferably, the RNA construct and/or lipid is exposed to a temperature of -60°C or lower, -65°C or lower, or -70°C or lower. Thawing may be achieved by exposing the RNA construct and/or lipid (or RNA/lipid nanoparticles) to a temperature of 0°C or higher, 5°C or higher, or 10°C or higher. Thawing may be achieved by exposing the RNA construct and/or lipid to a temperature of 15°C or higher, 20°C or higher, or 22°C or higher. Preferably, thawing occurs at room temperature. Thus, once the RNA construct and/or lipid (or RNA/lipid nanoparticles) has been frozen (preferably, at about -70°C), and then thawed (preferably, at room temperature), it is then preferably contacted with the carbohydrate (preferably, trehalose). Then, in one embodiment, the carbohydrate and any optional excipients are preferably added to create the vaccine formulation of the invention. In another embodiment, however, the carbohydrate may be added before the ethanol is removed. The vaccine formulation of the invention may be lyophilised or freeze-dried. Accordingly, the vaccine vial may comprise lyophilised vaccine formulation. The vaccine formulation may further comprise glycerol. The glycerol may be at a concentration of at least 1% (w/v), 2% (w/v), 3% (w/v), 4% (w/v) or 5% (w/v). Preferably, the glycerol concentration is at a concentration of at least 6% (w/v), 7% (w/v), 8% (w/v), 9% (w/v) or 10% (w/v). The vaccine formulation may comprise a metal chelator, such as sodium citrate and/or EDTA. Each dosage of the vaccine formulation may comprise between 0.1 and 20µg RNA construct, or between 0.5 and 15µg RNA construct. Preferably, each dosage of the vaccine formulation may comprise between 0.75 and 10µg RNA construct, or between 1 and 5µg RNA construct. A preferred dosage may be about 10µg RNA construct. Preferably, the vaccine vial or syringe of the fourth aspect comprises between 1 and 20 dosages, or between 2-10 dosages, or between 3-7 dosages of the vaccine formulation. More preferably, the vaccine vial or syringe comprises less than 20, 18, 16 or 15 dosages of the vaccine formulation. More preferably, the vaccine vial or syringe comprises less than 14, 12, 10 or 8 dosages of the vaccine formulation. The RNA construct may comprise any form of ribonucleic acid (RNA), and may be selected from a group consisting of: messenger RNA (mRNA); self-amplifying RNA (saRNA); siRNA and miRNA. Preferably, the RNA construct comprises mRNA or saRNA. Most preferably, the RNA construct comprises saRNA. mRNA is known as non-replicating mRNA, whereas saRNA may also be known as a self-replicating RNA virus vector, or an RNA replicon. The RNA construct may be double-stranded or single-stranded or a combination of double- and single-stranded RNA. Preferably, the RNA construct comprises self-amplifying RNA (saRNA), and is preferably a saRNA construct. Preferably, the RNA construct comprises or is derived from a positive-stranded RNA virus selected from the group of genus consisting of: alphavirus; picornavirus; flavivirus; rubivirus; pestivirus; hepacivirus; calicivirus or coronavirus. Suitable wild-type alphavirus sequences are well-known. Representative examples of suitable alphaviruses include Aura, Bebaru virus, Cabassou, Chikungunya virus, Eastern equine encephalomyelitis virus, Fort Morgan, Getah virus, Kyzylagach, Mayaro, Mayaro virus, Middleburg, Mucambo virus, Ndumu, Pixuna virus, Ross River virus, Semliki Forest, Sindbis virus, Tonate, Triniti, Una, Venezuelan equine encephalomyelitis, Western equine encephalomyelitis, Whataroa, and Y-62-33. Preferably, the RNA construct comprises or is derived from a virus selected from the group of species consisting of: Venezuelan Equine Encephalitis Virus (VEEV); enterovirus 71; Encephalomyocarditis virus; Kunjin virus; and Middle East respiratory syndrome virus. Preferably, the construct is derived from VEEV. The RNA construct comprises a sequence which encodes one or more antigen of interest. The at least one antigen of interest may be a protein or peptide derived from any human or animal pathogen, such as bacteria, viruses, fungi, protozoa and/or parasites. Preferably, the antigen of interest elicits an immune response in a vaccinated subject, including an antibody and/or T-cell immune response. Preferably, the antigen of interest is a protein and peptide derived from a virus, i.e. a viral antigen. The viral antigen may be derived from a virus selected from the group consisting of Orthomyxoviruses; Paramyxoviridae viruses; Metapneumovirus and Morbilliviruses; Pneumoviruses; Paramyxoviruses; Poxviridae; Metapneumoviruses; Morbilliviruses; Picornaviruses; Enteroviruseses; Bunyaviruses; Phlebovirus; Nairovirus; Heparnaviruses; Togaviruses; Alphavirus; Arterivirus; Flaviviruses; Pestiviruses; Hepadnaviruses; Rhabdoviruses; Caliciviridae; Coronaviruses; Retroviruses; Reoviruses; Parvoviruses; Delta hepatitis virus (HDV); Hepatitis E virus (HEV); Human Herpesviruses and Papovaviruses. The Orthomyxoviruses may be Influenza A, B and C. The Paramyxoviridae virus may be Pneumoviruses (RSV), Paramyxoviruses (PIV). The Metapneumovirus may be Morbilliviruses (e.g., measles). The Pneumovirus may be Respiratory syncytial virus (RSV), Bovine respiratory syncytial virus, Pneumonia virus of mice, or Turkey rhinotracheitis virus. The Paramyxovirus may be Parainkuenza virus types 1 - 4 (PIV), Mumps, Sendai viruses, Simian virus 5, Bovine parainkuenza virus, Nipahvirus, Henipavirus or Newcastle disease virus. The Poxviridae may be Variola vera, for example Variola major and Variola minor. The Metapneumovirus may be human metapneumovirus (hMPV) or avian metapneumoviruses (aMPV). The Morbillivirus may be measles. The Picornaviruses may be Enteroviruses, Rhinoviruses, Heparnavirus, Parechovirus, Cardioviruses and Aphthoviruses. The Enteroviruses may be Poliovirus types 1, 2 or 3, Coxsackie A virus types 1 to 22 and 24, Coxsackie B virus types 1 to 6, Echovirus (ECHO) virus) types 1 to 9, 11 to 27 and 29 to 34 or Enterovirus 68 to 71. The Bunyavirus may be California encephalitis virus. The Phlebovirus may be Rift Valley Fever virus. The Nairovirus may be Crimean-Congo hemorrhagic fever virus. The Heparnaviruses may be Hepatitis A virus (HAV). The Togaviruses may be Rubivirus. The Flavivirus may be Tick-borne encephalitis (TBE) virus, Dengue (types 1, 2, 3 or 4) virus, Yellow Fever virus, Japanese encephalitis virus, Kyasanur Forest Virus, West Nile encephalitis virus, St. Louis encephalitis virus, Russian spring-summer encephalitis virus or Powassan encephalitis virus. The Pestivirus may be Bovine viral diarrhea (BVDV), Classical swine fever (CSFV) or Border disease (BDV). The Hepadnavirus may be Hepatitis B virus or Hepatitis C virus. The Rhabdovirus may be Lyssavirus (Rabies virus) or Vesiculovirus (VSV). The Caliciviridae may be Norwalk virus, or Norwalk-like Viruses, such as Hawaii Virus and Snow Mountain Virus. The Coronavirus may be SARS CoV-1, SARS-CoV-2, MERS, Human respiratory coronavirus, Avian infectious bronchitis (IBV), Mouse hepatitis virus (MHV), or Porcine transmissible gastroenteritis virus (TGEV). The Retrovirus may be Oncovirus, a Lentivirus or a Spumavirus. The Reovirus may be an Orthoreo virus, a Rotavirus, an Orbivirus, or a Coltivirus. The Parvovirus may be Parvovirus B 19. The Human Herpesvirus may be Herpes Simplex Viruses (HSV), Varicella-zoster virus (VZV), Epstein-Barr virus (EBV), Cytomegalovirus (CMV), Human Herpesvirus 6 (HHV6), Human Herpesvirus 7 (HHV7), or Human Herpesvirus 8 (HHV8). The Papovavirus may be Papilloma viruses, Polyomaviruses, Adenoviruses or Arenaviruses. As shown in the examples, in a preferred embodiment, the viral antigen may be a Coronavirus antigen. Preferably, the Coronavirus antigen is a surface glycoprotein, more preferably SARS-CoV-2 surface glycoprotein. The protein and peptide derived from bacteria may be a bacterial antigen. The bacterial antigen may derived from a bacterium selected from the group consisting of: Neisseria meningitides, Streptococcus pneumoniae, Streptococcus pyogenes, Moraxella catarrhalis, Bordetella pertussis, Burkholderia sp. (e.g., Burkholderia mallei, Burkholderia pseudomallei and Burkholderia cepacia), Staphylococcus aureus, Haemophilus inkuenzae, Clostridium tetani (Tetanus), Clostridium perfringens, Clostridium botulinums, Cornynebacterium diphtheriae (Diphtheria), Pseudomonas aeruginosa, Legionella pneumophila, Coxiella burnetii, Brucella sp. (e.g., B. abortus, B. canis, B. melitensis, B. neotomae, B. ovis, B. suis and B. pinnipediae, Francisella sp. (e.g., F. novicida, F. philomiragia and F. tularensis), Streptococcus agalactiae, Neiserria gonorrhoeae, Chlamydia trachomatis, Treponema pallidum (Syphilis), Haemophilus ducreyi, Enterococcus faecalis, Enterococcus faecium, Helicobacter pylori, Staphylococcus saprophyticus, Yersinia enter ocolitica, E. coli, Bacillus anthracis (anthrax), Yersinia pestis (plague), Mycobacterium tuberculosis, Rickettsia, Listeria, Chlamydia pneumoniae, Vibrio cholerae, Salmonella typhi (typhoid fever), Borrelia burgdorfer, Porphyromonas s and Klebsiella sp. The protein and peptide derived from a fungus may be a fungal antigen. The fungal antigen may be derived from a fungus selected from the group consisting of Dermatophytres, including: Epidermophyton koccusum, Microsporum audouini, Microsporum canis, Microsporum distortum, Microsporum equinum, Microsporum gypsum, Microsporum nanum, Trichophyton concentricum, Trichophyton equinum, Trichophyton gallinae, Trichophyton gypseum, Trichophyton megnini, Trichophyton mentagrophytes, Trichophyton quinckeanum, Trichophyton rubrum, Trichophyton schoenleini, Trichophyton tonsurans, Trichophyton verrucosum, T. verrucosum var. album, var. discoides, var. ochraceum, Trichophyton violaceum, and/or Trichophyton faviforme; or from Aspergillus fumigatus, Aspergillus kavus, Aspergillus niger, Aspergillus nidulans, Aspergillus terreus, Aspergillus sydowi, Aspergillus kavatus, Aspergillus glaucus, Blastoschizomyces capitatus, Candida albicans, Candida enolase, Candida tropicalis, Candida glabrata, Candida krusei, Candida parapsilosis, Candida stellatoidea, Candida kusei, Candida parakwsei, Candida lusitaniae, Candida pseudotropicalis, Candida guilliermondi, Cladosporium carrionii, Coccidioides immitis, Blastomyces dermatidis, Cryptococcus neoformans, Geotrichum clavatum, Histoplasma capsulatum, Klebsiella pneumoniae, Microsporidia, Encephalitozoon spp., Septata intestinalis and Enterocytozoon bieneusi; Brachiola spp, Microsporidium spp., Nosema spp., Pleistophora spp.,Trachipleistophora spp., Vittaforma spp Paracoccidioides brasiliensis, Pneumocystis carinii, Pythiumn insidiosum, Pityrosporum ovale, Sacharomyces cerevisae, Saccharomyces boulardii, Saccharomyces pombe, Scedosporium apiosperum, Sporothrix schenckii, Trichosporon beigelii, Toxoplasma gondii, Penicillium marneffei, Malassezia spp., Fonsecaea spp., Wangiella spp., Sporothrix spp., Basidiobolus spp., Conidiobolus spp., Rhizopus spp, Mucor spp, Absidia spp, Mortierella spp, Cunninghamella spp, Saksenaea spp., Alternaria spp, Curvularia spp, Helminthosporium spp, Fusarium spp, Aspergillus spp, Penicillium spp, Monolinia spp, Rhizoctonia spp, Paecilomyces spp, Pithomyces spp, and Cladosporium spp. The protein and peptide derived from a protozoan may be a protozoan antigen. The protozoan antigen may be derived from a protozoan selected from the group consisting of: Entamoeba histolytica, Giardia lambli, Cryptosporidium parvum, Cyclospora cayatanensis and Toxoplasma. Preferably, the RNA construct comprises a sequence, which encodes the at least one innate inhibitor protein (IIP), which is capable of reducing or blocking the innate immune response to RNA in a subject treated with the vaccine formulation of the invention. The IIP is therefore an inhibitor of innate immunity. The reduction or blocking of the innate immune response to RNA is preferably achieved by the IIP by reducing or blocking recognition of RNA (preferably long RNA, which would be understood by the skilled person to mean RNA that is at least 1 kb in length) or dsRNA, by a host cell harbouring the RNA construct of the invention. More preferably, the innate inhibitor protein is an innate inhibiting protein such that it is capable of reducing or blocking the innate response to RNA, preferably the RNA of the RNA construct of the vaccine formulation. The RNA may be single stranded RNA or double stranded RNA. Preferably, the RNA is saRNA. The at least one innate inhibitor protein may be capable of either: (i) reducing or blocking the action of Melanoma Differentiation-Associated protein 5 (MDA5), for example by preventing oligomerization of MDA5 and binding of MDA5 to RNA, and/or (ii) blocking or reducing the binding of PACT to RNA, which may also be referred to as PKR activating protein, to RNA. The RNA construct may further comprise a 5’ cap. The term "5'-cap" includes a 5'-cap analogue that resembles the RNA cap structure and is modified to possess the ability to stabilize RNA and/or enhance translation of RNA if attached thereto, preferably in vivo and/or in a host cell. An RNA with a 5’-cap may be achieved by in vitro transcription of a DNA template in presence of said 5'-cap, wherein said 5'-cap is co-transcriptionally incorporated into the generated RNA strand, or the RNA may be generated, for example, by in vitro transcription, and the 5’ -cap may be attached to the RNA post- transcriptionally using capping enzymes, for example, capping enzymes of vaccinia virus. In capped RNA, the 3' position of the first base of a (capped) RNA molecule is linked to the 5' position of the subsequent base of the RNA molecule ("second base") via a phosphodiester bond. In one preferred embodiment, the RNA vaccine formulation comprises: (i) a lipid, which is preferably an LNP or liposome, (ii) an RNA construct (preferably a self-amplifying RNA construct) encoding an antigen of interest; and (iii) a carbohydrate or a pharmaceutically acceptable complex, salt, solvate, tautomeric form, stereoisomer or polymorphic form thereof, which is preferably trehalose. In a most preferred embodiment, the RNA vaccine formulation comprises a lipid, which is preferably an LNP, and which encapsulates a self-amplifying RNA construct encoding an antigen of interest, which is preferably a viral antigen, and trehalose. It will be appreciated that the RNA construct or vaccine formulation of the invention may be used in a medicament, which may be used as a monotherapy (i.e. use of the active agent), for treating, ameliorating, or preventing disease, or for vaccination, such as against a viral infection (e.g. coronavirus). Alternatively, the active agents according to the invention may be used as an adjunct to, or in combination with, known therapies for treating, ameliorating, or preventing disease or for vaccination. The RNA construct or vaccine formulation may be combined in compositions having a number of different forms depending, in particular, on the manner in which the composition is to be used. Thus, for example, the composition may be in the form of a powder, tablet, capsule, liquid, ointment, cream, gel, hydrogel, aerosol, spray, micellar solution, transdermal patch, liposome suspension, polyplex, emulsion, lipid nanoparticles (with RNA on the surface or encapsulated) or any other suitable form that may be administered to a person or animal in need of treatment or vaccination. It will be appreciated that the vehicle of medicaments according to the invention should be one which is well-tolerated by the subject to whom it is given. The RNA construct or vaccine formulation may also be incorporated within a slow- or delayed-release device. Such devices may, for example, be inserted on or under the skin, and the medicament may be released over weeks or even months. The device may be located at least adjacent the treatment site. Such devices may be particularly advantageous when long-term treatment with the RNA construct or vaccine formulation is required and which would normally require frequent administration (e.g. at least daily injection). In a preferred embodiment, however, medicaments according to the invention may be administered to a subject by injection into the blood stream, muscle, skin or directly into a site requiring treatment. Most preferably, the medicaments, including the RNA construct or vaccine formulation, are injected into muscle. Injections may be intravenous (bolus or infusion) or subcutaneous (bolus or infusion), or intradermal (bolus or infusion), or intramuscular (bolus or infusion). It will be appreciated that the amount of RNA construct or vaccine formulation that is required is determined by its biological activity and bioavailability, which in turn depends on the mode of administration, the physiochemical properties of the RNA construct or vaccine formulation and whether it is being used as a monotherapy or in a combined therapy. The frequency of administration will also be influenced by the half- life of the RNA construct or vaccine formulation within the subject being treated. Optimal dosages to be administered may be determined by those skilled in the art, and will vary with the particular the RNA construct or vaccine formulation in use, the strength of the pharmaceutical composition, the mode of administration, and the type and advancement of the viral infection. Additional factors depending on the particular subject being treated will result in a need to adjust dosages, including subject age, weight, gender, diet, and time of administration. Generally, a daily dose of between 0.001µg/kg of body weight and 10mg/kg of body weight, or between 0.01µg/kg of body weight and 1mg/kg of body weight, of the RNA construct or vaccine formulation of the invention may be used for treating, ameliorating, or preventing a disease, depending upon the active agent used. Preferably, the RNA construct or vaccine formulation may be given as a weekly dose, and more preferably a fortnightly dose, i.e. a first primer infection followed by a second booster injection about 2 weeks later. However, daily doses may be given as a single administration (e.g. a single daily injection or inhalation of a nasal spray). Alternatively, the RNA construct or vaccine formulation may require administration over several days, or even twice or more times during one day. As an example, the RNA construct or vaccine formulation may be administered as two (or more depending upon the severity of the disease being treated or immunised against) daily doses of between 0.07 µg and 700 mg (i.e. assuming a body weight of 70 kg). A subject receiving treatment may take a first dose upon waking and then a second dose in the evening (if on a two dose regime) or at 3- or 4-hourly intervals thereafter. Alternatively, a slow release device may be used to provide optimal doses of the RNA construct or vaccine formulation to a subject without the need to administer repeated doses. Known procedures, such as those conventionally employed by the pharmaceutical industry (e.g. in vivo experimentation, clinical trials, etc.), may be used to form specific formulations of the RNA construct or vaccine formulation and precise therapeutic regimes (such as daily doses of the agents and the frequency of administration). A “subject” may be a vertebrate, mammal, or domestic animal. Hence, compositions and medicaments according to the invention may be used to treat any mammal, for example livestock (e.g. a horse), pets, or may be used in other veterinary applications. Most preferably, however, the subject is a human being. A “therapeutically effective amount” of the RNA construct or vaccine formulation is any amount which, when administered to a subject, is the amount of the aforementioned that is needed to ameliorate, prevent or treat any given disease, or successfully immunise against a pathogen infection. For example, the RNA construct or vaccine formulation of the invention may be used may be from about 0.0001 mg to about 800 mg, and preferably from about 0.001 mg to about 500 mg. It is preferred that the amount of the RNA construct or vaccine formulation is an amount from about 0.01 mg to about 250 mg, and most preferably from about 0.01 mg to about 1 mg. Preferably, the RNA construct or vaccine formulation is administered at a dose of 1-200µg, more preferably 1-100µg, more preferably 1-50µg, and most preferably 1-25µg. A “pharmaceutically acceptable vehicle” as referred to herein, is any known compound or combination of known compounds that are known to those skilled in the art to be useful in formulating pharmaceutical compositions. In one embodiment, the pharmaceutically acceptable vehicle may be a solid, and the composition may be in the form of a powder or tablet. A solid pharmaceutically acceptable vehicle may include one or more substances which may also act as flavouring agents, lubricants, solubilisers, suspending agents, dyes, fillers, glidants, compression aids, inert binders, sweeteners, preservatives, dyes, coatings, or tablet- disintegrating agents. The vehicle may also be an encapsulating material. In powders, the vehicle is a finely divided solid that is in admixture with the finely divided active agents according to the invention. In tablets, the active agent (e.g. RNA construct or vaccine formulation) may be mixed with a vehicle having the necessary compression properties in suitable proportions and compacted in the shape and size desired. The powders and tablets preferably contain up to 99% of the active agents. Suitable solid vehicles include, for example calcium phosphate, magnesium stearate, talc, sugars, lactose, dextrin, starch, gelatin, cellulose, polyvinylpyrrolidine, low melting waxes and ion exchange resins. In another embodiment, the pharmaceutical vehicle may be a gel and the composition may be in the form of a cream or the like. However, it is most preferred that the pharmaceutical vehicle may be a liquid, and the pharmaceutical composition is in the form of a solution. Liquid vehicles are used in preparing solutions, suspensions, emulsions, syrups, elixirs and pressurized compositions. The RNA construct or vaccine formulation according to the invention may be dissolved or suspended in a pharmaceutically acceptable liquid vehicle such as water, an organic solvent, a mixture of both or pharmaceutically acceptable oils or fats. The liquid vehicle can contain other suitable pharmaceutical additives such as solubilisers, emulsifiers, buffers, preservatives, sweeteners, flavouring agents, suspending agents, thickening agents, colours, viscosity regulators, stabilizers or osmo- regulators. Suitable examples of liquid vehicles for oral and parenteral administration include water (partially containing additives as above, e.g. cellulose derivatives, preferably sodium carboxymethyl cellulose solution), alcohols (including monohydric alcohols and polyhydric alcohols, e.g. glycols) and their derivatives, and oils (e.g. fractionated coconut oil and arachis oil). For parenteral administration, the vehicle can also be an oily ester such as ethyl oleate and isopropyl myristate. Sterile liquid vehicles are useful in sterile liquid form compositions for parenteral administration. The liquid vehicle for pressurized compositions can be a halogenated hydrocarbon or other pharmaceutically acceptable propellant. Liquid pharmaceutical compositions, which are sterile solutions or suspensions, can be utilized by, for example, subcutaneous, intradermal, intrathecal, epidural, intraperitoneal, intravenous and particularly intramuscular injection. The RNA construct or vaccine formulation may be prepared as a sterile solid composition that may be dissolved or suspended at the time of administration using sterile water, saline, or other appropriate sterile injectable medium. The RNA construct or vaccine formulation of the invention may be administered orally in the form of a sterile solution or suspension containing other solutes or suspending agents (for example, enough saline or glucose to make the solution isotonic), bile salts, acacia, gelatin, sorbitan monoleate, polysorbate 80 (oleate esters of sorbitol and its anhydrides copolymerized with ethylene oxide) and the like. The RNA construct or vaccine formulation according to the invention can also be administered orally either in liquid or solid composition form. Compositions suitable for oral administration include solid forms, such as pills, capsules, granules, tablets, and powders, and liquid forms, such as solutions, syrups, elixirs, and suspensions. Forms useful for parenteral administration include sterile solutions, emulsions, and suspensions. All of the features described herein (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined with any of the above aspects in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. For a better understanding of the invention, and to show how embodiments of the same may be carried into effect, reference will now be made, by way of example, to the accompanying Figures, in which:- Figure 1 shows a schematic of how a self-amplifying RNA (saRNA) works; Figure 2 shows how saRNA provides enhanced antigen expression; Figure 3 shows the chemical structure of trehalose; Figure 4 shows the percentage of cells positive for expression of the SARS-CoV-2 spike glycoprotein as assessed by Flow cytometry 24 hours post transfection of HEK293T cells using material on the day of production; Figure 5 shows the percentage of cells positive for expression of the SARS-CoV-2 spike glycoprotein as assessed by Flow cytometry 24 hours post transfection of HEK293T cells using material stored for 2 weeks following production at 4 ⁰ C; and Figure 6 shows the percentage of cells positive for expression of the SARS-CoV-2 spike glycoprotein as assessed by Flow cytometry 24 hours post transfection of HEK293T cells using material stored for 2 weeks following production at 25 ⁰ C. Examples Materials and Methods Preparing vaccine formulation An saRNA construct encoding the spike protein of SARS-CoV-2 is prepared in an aqueous solution prior to mixing. The lipid components (cholesterol, DSPC, cationic lipid and PEGylated lipid) are dissolved in ethanol. The RNA and lipid solutions are rapidly mixed to form nanoscale particles. Once the particles have formed, the ethanol is removed by Tangential Flow Filtration, and the carbohydrate and excipients are added as part of this transfer into the final formulation. ELISA MaxiSorp high binding ELISA plates (Nunc) were coated with 100µl per well of 1µg ml ^-1 recombinant SARS-CoV-2 S protein with prefusion stabilised conformation in PBS. 3 columns on each plate were coated with 1:1000 dilution of goat anti-rat Kappa and lambda light chains. After overnight incubation at 4°C, the plates were washed 4 times with PBS-Tween 200.05% (w/v) and blocked for 1h at 37°C with 200µl per well blocking buffer (1% BSA-Tween-200.05% (v/v)). The plates were then washed and diluted mouse serum samples (1:100; 1:1000 and 1:10000) or a 5-fold dilution series of mouse IgG added (100µl). Plates were incubated for 1h at 37°C, then washed and secondary anti-mouse IgG-HRP antibody added (50µl). After incubation and washes, plates were developed using 50ul per well SureBlue TMB substrate and the reaction stopped after 5min with 50ul per well stop solution. The absorbance at 450nm was read on a spectrophotometer and antibody levels in rat serum determined by interpolation to the mouse IgG standard curve. Determining saRNA stability To determine the stability of the SARS-nCoV-2 saRNA vaccine at a low dilution in different storage conditions for a storage period of 1 and 2 weeks, and then 1, 2, 3, 4, 5, 6, 9, 12, 18, and 24 months. Sample Preparation The following conditions tested were: • Condition 1: 0.5 mg/mL stored at -70°C + 10% sucrose (100 mg/mL) • Condition 2: 0.5 mg/mL stored at 2-8°C + 10% sucrose (100 mg/mL) • Condition 3: 2 µg/mL stored at 2-8°C + 10% sucrose (100 mg/mL) • Condition 4: 2 µg/mL stored at 2-8°C + 20% sucrose (200 mg/mL) • Condition 5: 2 µg/mL stored at 2-8°C + 0.5% trehalose (5 mg/mL) • Condition 6: 2 µg/mL stored at 2-8°C + 1% trehalose (10 mg/mL) • Condition 7: 2 µg/mL stored at -20°C + 1% trehalose (10 mg/mL) • Condition 8: 2 µg/mL stored at -20°C + 10% glycerol • Condition 9: 2 µg/mL stored at RT + 10% sucrose (100 mg/mL) The original stock concentration of the LNP formulated SARS-nCoV-2 saRNA was at 0.5 mg/mL in a PBS buffer at pH 7.4 with 300mM sucrose. For conditions 3-9, the RNA was diluted at a 1:250 dilution in PBS. Then, the diluted RNA was mixed with the different additives at certain % according to each condition (below). Sucrose was dissolved in H ₂ O at a concentration of 1 g/mL (100% w/v) and trehalose was dissolved in H2O at a concentration of 50 mg/mL (5% w/v). The final volume of LNP formulated SARS-nCoV-2 saRNA with each additive for conditions 1-2 was 27 µL and conditions 3- 9 was 7 mL per timepoint. Therefore:- • Conditions 1 & 2: 2.7 µL of sucrose was added • Condition 3: 700 µL of sucrose was added • Condition 4: 1400 µL of sucrose was added • Condition 5: 700 µL of trehalose was added • Condition 6: 1400 µL of trehalose was added • Condition 7: 1400 µL of trehalose was added • Condition 8: 700 µL of glycerol was added • Condition 9: 700 µL of sucrose was added Example 1 The inventors tested a range of different RNA vaccine formulations to test if it is possible to thermally stabilise them at about 2-8°C. Referring to Figure 2, the inventors have adopted the self-amplifying RNA platform to develop a COVID-19 vaccine. Their chosen saRNA vector is based on a non-infectious Venezuelan Equine Encephalitis Virus (VEEV) replicon backbone encoding non- structural proteins required for the self-amplification step, where the SARS-CoV-2 S glycoprotein has been inserted in place of structural genes downstream of the subgenomic promoter (SGP). SaRNA is strictly confined to the cytosol, does not require a cDNA intermediary or penetration into the nucleus, and can generate very high expression of a gene product with a surprisingly low initial dose (i.e.1µg saRNA/subject). Formulated saRNA is taken up into the cytoplasm of target cells, which leads to intracellular amplification of the saRNA by the encoded polymerase machinery and very high expression levels of the viral antigen. The inventors tested a range of different saRNA vaccine formulations to see if it is possible to thermally stabilise them at about 2-8°C. Each vaccine formulation that was tested contained a self-amplifying RNA replicon encoding the SARS-CoV-2 spike glycoprotein antigen formulated in a standard liponanoparticle (LNP). The LNP was made of the structural lipids, cholesterol and DSPC, and the functional lipids (a PEGylated lipid and a cationic lipid), all of which encapsulated the RNA replicon. Nine different formulations (i.e. conditions) were tested, as set out below: • Condition 1: 0.5 mg/mL saRNA + 10% (w/v) sucrose stored at -70°C (control); • Condition 2: 0.5 mg/mL saRNA + 10% (w/v) sucrose stored at 2-8°C; • Condition 3: 2 μg/mL saRNA + 10% (w/v) sucrose stored at 2-8°C; • Condition 4: 2 μg/mL saRNA + 20% (w/v) sucrose stored at 2-8°C; • Condition 5: 2 μg/mL saRNA + 0.5% (w/v) trehalose stored at 2-8°C; • Condition 6: 2 μg/mL saRNA + 1.0% (w/v) trehalose stored at 2-8°C; • Condition 7: 2 μg/mL saRNA + 1.0% (w/v) trehalose stored at -20°C; • Condition 8: 2 μg/mL saRNA + 10% (w/v) glycerol stored at -20°C; and • Condition 9: 2 μg/mL saRNA + 10% (w/v) sucrose stored at room temperature. The immune potency of each of the above nine vaccine formulations (condition 1-9) was then tested after 1 and 2 weeks, and then at 1, 2, 3, 4, 5, 6, 9, 12, 18, 24 months. Potency was tested by injecting mice (n=4-5) with a 0.1 μg dose of each of the nine vaccine formulations (conditions 1-9). The mice were then bled after two weeks and then again after four weeks immunization with each formulation. Then, the concentration of IgG antibodies that were immunospecific to the SARS-CoV- 2 spike glycoprotein was determined using ELISA. Results & Discussion The inventors carried out the thermal stabilisation experiments for the various vaccine formulations consisting of self-amplifying RNA encoding the spike protein of SARS-CoV-2 encapsulated by the LNP. The inventors measured the immune response (measured as IgG titres against SARS-CoV-2) in subjects administered with the vaccine formulations denoted 1-9 after 2 weeks, 1 month, 2 months, 3 months, 4 months and 5 months storage duration. They found that, by month 5, the following conditions held their thermal stability: • Condition 1: 0.5 mg/mL saRNA + 10% (w/v) sucrose stored at -70°C (control) • Condition 5: 2 μg/mL saRNA + 0.5% (w/v) trehalose stored at 2-8°C • Condition 6: 2 μg/mL saRNA + 1.0% (w/v) trehalose stored at 2-8°C • Condition 7: 2 μg/mL saRNA + 1.0% (w/v) trehalose stored at -20°C • Condition 8: 2 μg/mL saRNA + 10% (w/v) glycerol stored at -20°C The inventors found that all of the formulations with sucrose (i.e. conditions 1-4 and 9) have lost their thermal stability. However, surprisingly, conditions 5 and 6 performed the best showing the most superior retention of thermal stability at 2-8°C. Conditions 5 and 6 both include the diglucose molecule, trehalose, at 0.5% (w/v) and 1.0% (w/v), respectively. Example 2 Previous experiments demonstrated that the addition of Trehalose following a freeze thaw step (-70 ⁰ C) provided prolonged product stability at 2-8 ⁰ C. Additional experiments were then performed to determine whether addition of Trehalose before freezing would provide protection on subsequent thaw of the product. Materials & Methods Method used for LNP formulated saRNA The nCoV saRNA encoding the SARS CoV2 spike glycoprotein was prepared by in vitro transcription and formulated in LNPs as previously described (ref 3). LNPs were composed of inoizable lipid (MC3; D-Lin-MC3-DMA; C ₄₃ H ₇₉ ;NO ₂ ; MWt: 642.1), DSPC, cholesterol, and DMPE-PEG2000 at a molar ratio of 50:10:38.5:1.5 at N/P = 14. The saRNA solution was prepared in 50 mM sodium acetate and 100 mM sodium chloride buffer at pH = 5.5. LNPs were formulated on a NanoAssemblr (Precision NanoSystems Incorporated, Vancouver, BC, Canada) using a flow rate ratio of RNA:lipid of 3:1 and a total flow rate of 8 mL/min. Formulated material was then concentrated in DPBS containing the relative concentration of excipient in approximately 5 times the volume of total formulation, and filtered using VivaSpin 6 tubes with a molecular weight cut-off (MWCO) of 10 kDa. Method used for the Flow potency assay HEK293 cells were transfected with LNP formulated saRNA encoding the nCoV (SARS- CoV-2) spike glycoprotein in 12-well plate for 24 h. The pDNA control was transfected using lipofectamine 3000. Each RNA condition was transfected in triplicates at 1ug. The next day, all cells were collected following trypsin treatment and centrifuge at 1750 rpm for 7 min. The supernatant was discarded and cells resuspended in 100 μL of FACS buffer (PBS with 2.5% FCS).50 μL of LIVE/DEAD™ Fixable Aqua Dead Cell Stain (Invitrogen, Cat no. L34957) was added in a 1:400 dilution and cells incubate samples on ice for 20 min. Cells were then washed with 2.5 mL FACS buffer and centrifuged at 1750 rpm for 7 min. Primary monoclonal antibody to the CoV2 spike protein was added to the cells at a concentration of 10ug/mL in 50 μL FACS buffer, cells were vortexed and incubated on ice for 30 min. Cells were washed with 2.5 mL FACS buffer and centrifuged at 1750 rpm for 7 min. Secondary detection antibody: 2 μL of Mouse IgG1 Anti-Human IgG AF647 in 50 μL FACS buffer was added to cells, vortexed and incubated on ice for 30 min. Subsequently cells were wash with 2.5 mL FACS buffer and centrifuged at 1750 rpm for 7 min to pellet the cells. These were then resuspend in 250 μL of PBS only, vortexed and 250 μL of 3% paraformaldehyde was added as fix solution (final fixation is 1.5%). Cells were vortexed and filtered before analysis by Flow cytometry. Samples were run on the flow cytometer using 405 nM laser (collection filter: 525/50) for the Live/Dead Fixable Aqua Dead Cell stain and 640 nM laser (collection filter: 670/14) for the Mouse IgG1 Anti-Human IgG AF647 stain. 10,000 events were collected and expression levels evaluated according to the gating of untransfected cells. References 1. Biondi AC, Senisterra GA, Disalvo EA. Permeability of lipid membranes revised in relation to freeze-thaw processes. Cryobiology.1992;29(3):323-31. 2. Ragoonanan V, Wiedmann T, Aksan A. Characterization of the effect of NaCl and trehalose on the thermotropic hysteresis of DOPC lipids during freeze/thaw. J Phys Chem B.2010;114(50):16752-8. 3. McKay PF, Hu K, Blakney AK, Samnuan K, Brown JC, Penn R, Zhou J, Bouton CR, Rogers P, Polra K, Lin PJC, Barbosa C, Tam YK, Barclay WS, Shattock RJ. Self-amplifying RNA SARS-CoV-2 lipid nanoparticle vaccine candidate induces high neutralizing antibody titers in mice. Nat Commun.2020;11(1):3523. Results & Discussion Experimental data demonstrated that while the formulated product was active before freeze-thaw treatment in the absence or presence of trehalose (see Figure 1), that two weeks following thaw and storage of material at 4 ⁰ C (see Figure 2) or 25 ⁰ C (see Figure 3) the product had no activity above background when compared to material formulated with 10% sucrose, stored at -70 ⁰ C and thawed on the day of transfection. These data indicate that the surprisingly protective effects of trehalose require a freeze shock treatment (i.e. freeze-thaw) and addition of the excipient post thaw. This was totally unexpected, that is to say, the surprising discovery that freeze-thaw treatment is required for trehalose to provide product stabilising activity. Although they do not wish to be bound by any hypothesis, the inventors believe that the freezing-thawing mechanism indicates a change in the product that allows trehalose to interact with the product (LNP formulated saRNA) in a manner that is different to product that has not been subjected to a freeze shock step. In this respect, it is known that the freeze-thaw process can affect the phase behaviour, curvature and surface properties of lipid bilayers (refs 1 & 2). Summary RNA vaccines normally suffer the drawback of poor thermal stability, which is why they require storage at temperatures of -20°C or even -70°C, in highly specialised freezing equipment. However, the inventors have demonstrated that a lipid-based (e.g. LNP) vaccine formulation comprising very low concentrations of RNA (e.g.2µg/ml RNA) is thermally stabilised by 0.5% or 1.0% (w/v) trehalose, i.e. a weight ratio of RNA:trehalose of 1:2,500 or 1:5,000, respectively. Being able to thermally stabilise a vaccine formulation having low concentrations of RNA is particularly advantageous when preparing low dose vaccine vials (such as saRNA replicon vaccines), because it means that fewer doses (e.g. about 1-10 doses) can be contained in a single vial and safely stored 2-8°C. It will be appreciated that this significantly improves the logistics of a mass immunisation program using such RNA vaccines, because standard freezers can be used instead of the specialised freezers required to maintain RNA vaccines at - 20°C or -70°C. This significantly reduces waste that would otherwise occur if much higher dose vaccine vials are used. In addition, the inventors very surprisingly found that these protective effects of trehalose require a freeze shock treatment (i.e. freeze-thaw) and addition of the trehalose post thaw rather than addition of trehalose before freezing. This was not expected.