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Clearing up the confusion about COVID boosters

A distinction needs to be drawn between people who may not have had a proper immune response and immunity that wanes over time.

This article was originally published in the Montreal Gazette

There has been much discussion recently about whether we will need vaccine boosters to protect us as we head into the fall, especially with the increase in the Delta variant’s transmission. The issue of when and if boosters would be necessary has been debated for months, but it took on new importance when the U.S. Centers for Disease Control and Prevention recently recommended a third dose for people who are immunocompromised.

We first need to clarify some definitions that are sometimes used interchangeably even though they do not mean the same thing. First is the issue of whether we are talking about a third dose of the same vaccine that everyone has already received, or about a different vaccine specifically directed against the new variants.

There has always been a concern that the vaccines might not be effective against something like the Delta variant. Remember that the vaccine is prompting your immune system to make antibodies against the spike protein of the virus. As the virus mutates, and the spike protein changes, it is possible that the antibodies generated post-vaccination would not recognize the spike protein of the mutated virus and therefore not provide you with adequate protection. Fortunately, that has not happened yet, and recent data has shown us that a double dose of the Pfizer vaccine is still very effective (between 85 and 90 per cent) against the Delta variant. However, it is possible that at some point in the future we might need a new vaccine specifically directed against a new variant, much as the flu vaccine is a different vaccine every year, based on the dominant circulating strain of that flu season.

The current issue is a bit different, as the CDC is talking not about a new vaccine, but about giving an additional dose to people who are immunocompromised and might not have had a proper immune response during their initial vaccination. Here, too, a distinction needs to be drawn between this problem and immunity that wanes over time. There has always been a concern that immunity from the vaccinations would fall with time and that a booster would be required. That does not seem to be the case so far, with Pfizer and Moderna announcing that at six months, vaccine efficacy was 84 and 93 per cent, respectively. While these represent small dips from peak efficacy right after vaccination, the vaccines are reassuringly still highly effective six months out.

The issue being debated is whether people who are immunocompromised should get a third dose of an mRNA vaccine because they might not have had an adequate immune response to begin with. A recent study in transplant patients showed that many such patients did not generate a robust immune response post-vaccination because they were taking anti-rejection medication that suppressed their immune system. But a third dose triggered antibody formation in those who had none and boosted antibody levels in those whose levels were low. Based on this and other data, the CDC changed its advice recently to recommend an additional dose for people who are immunocompromised or actively taking immune-suppressive medication.

This might seem to be at odds with the World Health Organization’s position that rich countries should forgo giving booster shots to their populations and redirect doses to countries without access to vaccines. But the WHO was arguing against boosters for waning immunity, which does not seem to be a major concern yet, rather than against giving an additional dose to immunocompromised individuals. Their argument is that vaccinating the unvaccinated is of greater benefit to the planet than a booster in otherwise healthy fully vaccinated individuals.

The who, what and when of additional vaccine doses remains to be seen, but the current CDC recommendations do not mean, as some have claimed, that we will all need boosters. While that might change with time, for most of us two doses will do just fine right now.


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