They probably don’t need a fourth Covid shot

A woman receives a booster shot of the Moderna Covid-19 vaccine on November 10, 2021 in Palm Harbor, Florida.


Chris Urso/Tampa Bay Times/Zuma Press

How many Covid vaccinations are enough? Pfizer and Moderna have asked the Food and Drug Administration to approve a second booster shot (a fourth injection) for patients over 65 and all adults, respectively. The FDA will reportedly approve (but not recommend) the fourth vaccine for patients over 50. But if your immune system is healthy, three or even two doses of these mRNA vaccines should be enough.

Vaccine-induced protection against infection is short-lived and doesn’t get much of a boost from extra shots. Still, the initial two-dose regimen is sufficient to provide most patients with excellent protection against serious disease – mediated by sustained cellular responses, not the neutralizing antibodies that rise and fall rapidly after vaccination.

The New York State Health Department’s large database shows that the efficacy of full vaccination (ie, at least two mRNA doses) versus hospitalization remained above 90%, even during the recent omicron surge. A study from Sweden found the same thing. Studies from Qatar and California showed no decrease in protection against serious illness with Omicron.

Supporters of the booster shot point to other studies showing diminishing vaccine effectiveness over time, particularly against Omicron. However, these appear to be unreliable, reporting a range of results for vaccine efficacy against symptomatic disease from as high as 40%-50% to as low as possible negative 40%

When evaluating vaccine efficacy, scientists compare disease rates between vaccinated and unvaccinated groups; a larger difference indicates higher vaccine efficacy. But as more and more unvaccinated people become immune from infection — particularly a highly transmissible variant — the gap between groups is narrowing, even when vaccines are still effective.

What about studies reporting lower rates of Omicron hospitalizations in boosted patients compared to unboosted patients? The reduced virulence of the variant means that a much lower percentage of those who get the vaccine end up in the hospital. Those who do are more likely to have serious underlying health problems. Raise The population helps. But for immunocompetent people with one or no risk factor, two doses remain 95% effective in preventing severe Covid.

Evidence for a fourth dose is even less convincing. The largest Israeli study reported that less than 1 in 200 people over 60 who received Omicron developed serious illness after three doses. A fourth dose further reduced that likelihood, but the effect was driven by those with major risk factors. There is no evidence that the introduction of boosters last fall had a significant impact on the trajectory of the US Omicron surge

If you have received two doses of vaccine you have good protection against severe Covid. Likewise if you have been infected with the virus, including Omicron. If you are over 65 or otherwise at high risk of serious illness, it makes sense to get a third dose. A fourth dose has already been approved for immunocompromised people. For everyone else, the data hasn’t shown any meaningful benefit from three doses, let alone four.

dr Krause is an advisor to the World Health Organization and served as deputy director of the FDA’s Office of Vaccines Research and Review from 2011 to 2021. dr Borio is a senior fellow for global health at the Council on Foreign Relations and was the director of preparedness policy for medicine and biodefense at the National Security Council from 2017-2019.

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Appeared in the print edition of March 29, 2022. They probably don’t need a fourth Covid shot

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