If you've had two doses of either mRNA vaccine, you technically remain immunized against COVID-19, emphasizes Jessica Malaty Rivera, MS, Science Communication Lead at the COVID tracking project. Booster shots, however, are being considered because we know that vaccine efficacy diminishes over time. Officials have been waiting on data to suggest when protection diminishes enough to warrant a third shot. We now have that data.
According to Timothy Brewer, MD, professor of medicine and epidemiology at UCLA, the results of two new studies informing this decision were released last week. The first study looked at mRNA vaccine effectiveness in New York state jab recipients from May-July, 2021, and found that vaccine effectiveness in preventing mild-to-moderate disease declined from 91.7 percent to 79.8 percent in that time period. He notes, however, that mRNA vaccines remained 91.9 to 95.3 percent effective in preventing hospitalization.
The second study looked at nursing home residents (a vulnerable population) across the country and found that vaccine effectiveness in preventing infection decreased from 74.7 percent early in the vaccination program (March-May 2021) to around 53.1 percent once the Delta variant became dominant (June-July 2021).
And Rivera says a third study out of Israel showed that antibody levels in individuals inoculated with the Pfizer vaccines began declining during the five, six, seven, and eight-month mark. In fact, efficacy against all infection was 95 percent in the January to April time period but fell substantially in late June and early July. The shot remained 90 percent effective in preventing severe illness throughout the entire timeline, however. (It's worth noting that this study is caveated as being small and measuring a relatively short period of time.)
You'll notice that none of these studies definitively point to eight months as the point in time at which antibodies significantly decrease, and Rivera points out that at no point in time will your antibody levels fall off a cliff. Both epidemiologists point out that you remain well-protected after two shots, especially against hospitalization and death.
The reason for the third shot is simply to provide extra protection against severe symptomatic illness in light of the extremely transmissible Delta variant, says Rivera. "There is some data to show that adding an additional dose can increase the immune response significantly," she says. "These vaccines are extremely good at keeping people out of the hospitals and keeping people alive, we just want to make sure that people who do have breakthroughs have the mildest of infections, preferably lowest to no symptoms, and it seems like an additional dose can do that."
And booster doses are not unique to COVID-19 vaccines, says Rivera. "This is not an expression of failure," she says. "This is a very very normal thing to happen in the progression of an infectious disease outbreak—not to mention the fact that the virus itself is now a little bit different [due to the Delta variant] and we have to pivot [by further bolstering immunity with a third dose]."
But Rivera also reiterates that the efficacy and necessity of boosters still need to be evaluated and recommended by the FDA and CDC before additional doses will be officially recommended in non-immunocompromised populations—so contrary to how headlines may make it seem, booster rollout is not yet set in stone.
And Dr. Brewer points out that boosters aren't actually the number one priority right now in terms of the global fight against COVID-19. "The challenge that a number of epidemiologists and public health researchers like myself have with the current recommendation is that if we want to prevent the most disease and help the most people we should be focusing on getting unvaccinated people vaccinated," says Dr. Brewer. "People need to remember that this is a global pandemic, and if there's transmission anywhere in the world, that creates the opportunity for new variants to put all of us at risk, and so we need to be thinking about how we get vaccine everywhere it's needed, as quickly as possible, and I think boosters detract from that a little bit."
Still, Rivera says that this isn't an either/or proposition. "There needs to be a very strict prioritization [with boosters], which is first immunocompromised people, then the elderly, then long-term care facility residents, and pregnant people, and high-risk people," she says. "But I believe very strongly that it has to be both—it's not boosters or helping the world, it has to be both."
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