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/

SARS-CoV2 (Continued)

SARS-CoV2 (Continued)  

     One of the puzzling features of this virus is that some infected people, while carrying a considerable viral load, and shedding infectious virions, nevertheless develop such trivial symptoms that they never notice they are infected. While other, of course, develop virally driven hyper-inflammation,  respiratory failure, and sometimes also kidney and heart failure.

     This variability in response is especially striking when it affects a whole nation-state. Thus the official WHO figures record that Vietnam, with a population of 97 million, has confirmed only 416 cases of COVID-19, and that none have died. (c.f. UK, population 67 million, 297,914 cases, 45,677 deaths on 24th July. )

Possible explanations for variable responses.

     The hypothesis of genetic variability in the human host, which supposes that the Vietnamese lack e.g. the ACE2 receptor site (See my SARS-CoV2 post), is more-or-less ruled out by anecdotal observations such as that of an asymptomatic carrier infecting 5 family members [1]. As also is the hypothesis of genetic variability among the circulating SARS-CoV2 strains, for the carrier would obviously infect the household with the strain she was carrying. 

     Could there be competition between two co-infecting viral strains, where one causes trivial, often negligible, symptoms but occupies all the binding sites? 

     Or could there be, in some people, residual anti-bodies at a sufficient titre from a previous infection by the same (or sufficiently similar) coronavirus?  This last seems the best hypothesis, and in the last 10 days has received some support. 

    The group of Antonio Bertoletti at the Duke-NUS Medical School in Singapore has just published in Nature [2] an online report showing that previous infection with a virus of the beta-coronavirus family can leave long-lasting and multispecific T cell immunity to the nucleocapsid structural protein (N protein, or NP, See my Coronavirus post) that can cross-react with the N protein of SARS-CoV2). This previous infection could be a harmless "common cold" member of the corona virus family, but in Singapore it was possible also to study survivors of the 2003 SARS pandemic. 

     Back in 2013 a group in Taiwan explored the antigenicity of the N protein of the mild common cold virus HCoV-OC43, and had found that the middle section was highly antigenic [3]. Well over 90% of healthy young adults contained antibodies in their serum against the N protein of this common virus. These antibodies were even found in cord-blood samples showing that newborn babies acquire some immunity against coronaviruses from their mothers. 

     We have already learnt that it is foolish to infect yourself deliberately with SARS-CoV2; you could become very ill or die. But there may be a beta-coronavirus, prevalent in Vietnam, that does protect you against COVID-19. And it may be that here in Britain a sufficiently recent 'common cold' may leave you with enough circulating antibodies to prevent or greatly limit the effect of SARS-CoV2 infection. 

References

[1]  Susan Lee,  Paula Meyler,  et al. (2020) Can J Anaesth. : 1–7. "Asymptomatic carriage and transmission of SARS-CoV-2: What do we know?". 

[2]  Le Bert N, Tan AT, Kunasegaran K, et al. (2020)  "SARS-CoV-2-specific T cell immunity in cases of COVID-19 and SARS, and uninfected controls." [published online ahead of print]. Nature. 2020;10.1038/s41586-020-2550-z. doi:10.1038/s41586-020-2550-z.  

See also  Ruairi J. Mackenzie, Science Writer for Technology Networks (2020) “'Common Cold' Coronaviruses Could Help Produce Anti-SARS-CoV-2 Immune Cells." 

[3]  Fang-Ying Liang, Leng-Chieh Lin, (2013  J Virol Methods;187(2):413-20. "Immunoreactivity characterisation of the three structural regions of the human coronavirus OC43 nucleocapsid protein by Western blot: implications for the diagnosis of coronavirus infection. "