13 November 2020

RNA Vaccines

RNA Vaccines

Currently (12 Nov. 2020), the World Health Organisation (WHO) is aware of 

48 different teams around the world who are working on the production of vaccines against the SARS-Cov2 virus that have already got to the stage of clinical evaluation. The Pfizer/BioNTech/FosunPharma team, known to all since 9th November when it announced a degree of success, is one of these 48. (There are in addition 160 other vaccine-production teams that are in pre-clinical stages of development.)

A number of different vaccine technologies are being tried in the 48 different vaccine-teams that are already at the stage of clinical trials, listed by WHO [1].  I summarise these below. 


Type 1. Inactivated virus (in this case inactivated SARS-Cov2). This is the approach used in the Salk polio vaccine, which used formaldehyde to ‘kill’ the virus. Formaldehyde can modify the shape of proteins, and the antibodies produced may only react with formaldehyde-treated virus. The virus must be really, really, dead, and safety is a perpetual concern.


Type 2. Replicating viral vectors. Thus, the SARS-Cov2 spike protein gene can be inserted into the genome of a mild virus (e.g. the measles virus or adenovirus). These viruses have their own way of getting into cells and replicating, but introduces a SARS-Cov2 antigen, against which the host can raise antibodies. (See [2]


Type 3. Non-replicating viral vectors. Adenoviruses often used.These can get into cells but will not spread in the host. Higher doses are therefore needed. (This is the strategy used by the Oxford/Astra Zeneca team.) 


Type 4. DNA vaccines. The mRNA for a viral gene is copied (using reverse transcriptase) into a double-stranded DNA plasmid that grows happily in bacteria. Large quantities of the plasmid are grown up, purified on columns and used as vaccine. Once inside a cell they should direct the synthesis of e.g. Spike protein (amongst several others.)


Type 5. Protein subunit vaccines. These are a more recent development, and becoming popular, as no virus is involved in the manufacture. The gene for a viral protein can be used to produce large quantities of the protein in vitro. However, the isolated and purified protein may not have the right shape to trigger formation of antibodies effective against native virus. 


Type 6. Virus-like Particles (VLPs) can be prepared by growing cultured cells that produce only sufficient of the viral proteins to form a particle, but are not able to reproduce whole virus. If RNA is needed to form a particle, small bits of irrelevant RNA can be added. These particles, purified from cell cultures, can be used as vaccines, and are often more potent antigens than the isolated soluble protein or protein subunits of type 5. Again, no virus is involved in the process of manufacture.


Type 7. RNA vaccines. In this strategy single-stranded mRNA that codes only one viral protein (e.g. the Spike protein) is encapsulated in a Lipid Nano Particle (LNP) some 70-100 nm in diameter [3] (1million nm = 1 mm). Human cells have an inherent tendency to engulf particles of a particular size and attempt to digest them (a hangover, no doubt, from our amoeboid ancestry). The released mRNA directs the synthesis of spike protein (or its Receptor Binding Domain) in the cell. This technology has been developed over the last 20 years for experimentally silencing genes; and since 2012 for producing vaccines against single-strand RNA viruses such as influenza. It was first used in humans in 2017 [4]. The advantage is that an equipped factory can turn to producing a novel vaccine within a week. All it needs is to know the sequence of the mRNA. (This is the strategy use by the Pfizer/ BioNTech/ FosunPharma team, and a team at Imperial College, London.) RNA is far more susceptible to hydrolysis than either protein or DNA (because of the -OH, group missing in 2' desoxyribose). Vaccines are conventionally kept at 5-8ºC, but RNA vaccines must be kept at –78ºC or lower. That is not a problem. A 6 litre dry-ice or liquid nitrogen Dewar, twice the size of a pressure cooker, will hold its temperature for 200 days.


We see that the different vaccine strategies have their own advantages and disadvantages. The RNA technology has the advantage of speed; and relative safety. 

References

[1]    https://www.who.int/publications/m/item/draft-landscape-of-covid-19-candidate-vaccines

[2]    https://doi.org/10.1016/j.virol.2014.01.002 

[3]    https://www.liebertpub.com/doi/full/10.1089/nat.2018.0721

[4]    https://pubmed.ncbi.nlm.nih.gov/28457665/



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