ALTHOUGH LOCKDOWNS can keep the coronavirus at bay, vaccination provides the sustainable path out of the pandemic. More than 60 vaccines are either in development or current use against SARS-CoV-2. All of those in use have the same ultimate result—of granting the body an enhanced ability to fend off viral attack—but the mechanisms they use differ considerably.
When the body becomes infected with a virus that it has never encountered before, the immune system begins a process of producing attack cells that can potentially destroy the intruder. This process takes both time and energy, because it involves considerable trial and error. It is one reason why people feel exhausted and ill for many days after contracting a new infection. If the body wins, the immune system remembers its successful strategy, so that future battles take much less time and symptoms are milder, or even non-existent.
Vaccines are, in effect, military training programmes for the immune system. Rather than forcing the immune system to learn how to solve a problem during an actual attack, a vaccine sets up a mock attack for the body to practise on. Vaccines commonly use four types of mock attack, all of which are being deployed against SARS-CoV-2.
The oldest of these techniques is to present the immune system with the virus in a form that has been inactivated or significantly weakened, so that it cannot cause a full-blown infection. When immune cells detect such an intruder, they still engage in the arduous process of generating attack cells. This is why people often feel tired for a day or two after vaccination. Yet the immune system develops a memory of the event, which enables it to act quickly should it later suffer an attack from a similar virus. Both the Chinese Sinovac and Sinopharm vaccines use an inactivated virus.
An alternative tactic involves injecting protein fragments that are structurally similar to SARS-CoV-2. These proteins are detected as foreign by the immune system, and hence trigger an immune response, but they are not capable of initiating an invasion on their own. Novavax, an American developer, is using this technique in its covid-19 vaccine.
A more complex strategy involves inserting some of the DNA copied from SARS-CoV-2 into a version of a virus related to the common cold (known as an adenovirus) that can enter cells but has been neutered and cannot replicate effectively. Once inside the body, these engineered viruses stick the adenovirus DNA containing the copy of the SARS-CoV-2 DNA into cellular machinery that then leads these infected cells to produce lots of SARS-CoV-2 viral proteins and stick them on their surfaces so that the immune system spots that something is awry. Crucially, the engineered virus also triggers an intruder alert system within these cells that draws immediate attention from the immune system. This stimulates the production of attack cells, creating an immune memory. The AstraZeneca/Oxford vaccine and the Gamaleya vaccine developed in Russia both use this strategy.
The newest method involves researchers creating genetic instructions, in the form of RNA or DNA, that cause the body’s own cells to generate harmless proteins that look like the distinctive proteins of SARS-CoV-2. Like the AstraZeneca and Gamaleya tactic, this method is potent because the RNA/DNA strategy turns human cells into viral protein production-centres that are all but certain to generate enough of the SARS-CoV-2 protein to trigger detection and hence to prime the immune system. This is not always the case with older vaccine technologies. The drawback to this technique is that DNA/RNA strands are transported into the body in capsules that, though easily absorbed by cells, are exceptionally fragile and often require storage at very low temperatures. Both the Pfizer/BioNTech and Moderna/NIAID vaccines use this strategy.
Practically, all the vaccines being used give the body a much better chance of mounting a successful defence against SARS-CoV-2 than it could mount if left to fend for itself. As for how well the different techniques can cope with the virus as it evolves, every one of them will need to be tweaked as the circulating virus changes significantly (as it already has done in variants found in both Britain and South Africa). The ease of tweaking, however, varies. Although RNA/DNA vaccines are the most cumbersome to transport and store, they are also the easiest to tinker with and may ultimately prove to be the most important if covid-19 evolves rapidly enough to evade the protection afforded by the current crop of vaccines.