As the planet staggers into the second year of living with the novel coronavirus, all hope for a relatively swift return to normal is being pinned on the prospect of mass vaccination. It appears clear that herd immunity was a chimera and that the virus is not going to simply peter out as some do, leaving vaccines as the only realistic means of resolving the crisis.
Globally, more than a hundred laboratories and companies are working on various forms of vaccine, but we will focus on the technology behind the three most notable ones produced to date, the Moderna vaccine, the Pfizer vaccine, and the Oxford AstraZeneca vaccine. These are the three that have most rapidly gained approval for mass dissemination and have become the primary vaccines for North America and Western Europe, as well as elsewhere (Russia and China are developing their own variants, but information about the composition of these and their effectiveness is currently scant).
The Pfizer and Moderna vaccines are similar in that they both use mRNA (modified RNA) as the basis for their vaccine (in fact Moderna is an abbreviation of modified RNA). Research into mRNA has been ongoing for the last three decades, although no mRNA vaccine, apart from these two, has been approved for human use. Unlike conventional vaccines, mRNA vaccines do not contain any trace of the virus they are designed to combat; instead, they contain genetic material that triggers the immune system into producing antibodies against the virus. Original research into the use of RNA in human therapy was hampered by the fact that in experiments with mice a degree of fatality was found; this was overcome by University of Pennsylvania scientists in 2005, who took one of the four building blocks of RNA and replaced it with pseudoridine, a modification that rendered it safe for therapeutic use.
The Oxford AstraZeneca vaccine is a more conventional offering that works in much the same way as common vaccines that we have all taken for smallpox, measles etc, i.e. it introduces a small quantity of the virus being fought (or a similar virus that provokes the same immune response), triggering the immune system to produce antibodies that are ready to fight off the virus when it is encountered. In this instance, the Oxford researchers have used ChAdOx1, a weakened adenovirus that causes a mild cold in chimpanzees. This virus has been genetically modified to ensure that it cannot expand when introduced into human subjects and overwhelm the immune system, which makes it safe for use in elderly patients, children, and those with underlying conditions such as diabetes. The vaccine imitates the spikes on the surface of the coronavirus (familiar to us all from illustrations of the virus) and provokes an immune reaction, the antibodies of which can then attack the coronavirus through its surface spikes when it is encountered.
There has been considerable confusion regarding the efficacy of these different forms of vaccine, how protective each one is after the first dose and the second dose, whether the different vaccines can be combined, and so forth. The current consensus appears to be that combining vaccines is inadvisable and that they all offer a roughly similar level of protection against the virus of between 90% and 95%. It should be remembered that these vaccines are intended to combat the virus when it is encountered, so an individual who has had the vaccine may be carrying it asymptomatically and is still capable of transmitting it to others.
What vaccine a country chooses to offer its citizens will depend on many factors, including cost and availability. The mRNA vaccines are considerably more expensive (Moderna £28 per dose, Pfizer £15) than the Oxford AstraZeneca vaccine (£3 per dose) and require more careful handling (Moderna storage at -30°C, Pfizer -70°C, Oxford AstraZeneca -2°C to +8°C). Nationalism may play its part, with the USA favouring the home-produced mRNA vaccines and the UK being more enthusiastic about the Oxford AstraZeneca vaccine. However, it is to be hoped that choices will principally be governed by what represents the most workable situation for a particular environment and the greatest likelihood of protecting the population at large in the quickest and most effective manner possible.