The Food and Drug Administration authorized a third dose of the Pfizer-BioNTech Covid-19 vaccine on Wednesday for emergency use in people ages 65 and up, as well as those ages 18 and up at high risk of exposure to the coronavirus or severe illness. Boosters are to be given at least six months after a person receives their second dose of the Pfizer vaccine.
The FDA's decision came as an advisory group to the Centers for Disease Control and Prevention held a two-day meeting on the safety and effectiveness Pfizer's booster shot. On Thursday, those advisers will vote on who would qualify for a third shot.
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Recommendation from the CDC group, called the Advisory Committee on Immunization Practices, is needed before booster shots can be administered. If the head of the CDC, Dr. Rochelle Walensky, signs off on the committee's recommendation, third doses of the Pfizer vaccine could be administered promptly.
The timing of the FDA authorization cast an air of uncertainty over the first day of the ACIP meeting, which usually knows at the outset what FDA decision it will be considering. Instead, the FDA announcement did not come until after the meeting concluded for the day Wednesday.
Pfizer originally asked the FDA to consider a third dose for everyone ages 16 and older six months after they received their second dose. Last Friday, an FDA advisory committee rejected that request, limiting the booster to older adults and those most likely to become seriously ill from Covid-19.
The FDA advisory committee also suggested that people at high risk of exposure at work, such as health care workers and teachers, should also be eligible for a booster dose.
That was reflected in Wednesday's EUA. "Health care workers, teachers and day care staff, grocery workers and those in homeless shelters or prisons, among others" will be able to receive a third dose of the Pfizer vaccine, acting FDA commissioner Dr. Janet Woodcock said in a statement.
Additional data on the safety and effectiveness of boosters is expected in coming weeks. "We will continue to evaluate the rapidly changing science and keep the public informed," Woodcock said.
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The booster dose is aimed to combat waning immunity from the original series of the vaccine. Data from Israel, presented at Friday's advisory committee meeting, suggests that protection from Pfizer's vaccine drops off significantly after six months, though data from the United States show less of a decline, particularly against severe disease. Early data from Israel also showed that a booster dose offered more protection against infection.
In rejecting Pfizer's original request — that a booster be made available to everyone ages 16 and older — the FDA committee cited concerns about the shot's possible side effects, such as myocarditis, in younger adults.
ACIP members also struck a skeptical tone for much of Wednesday’s discussions on booster doses. In addition to requests for additional data, committee members wanted more clarity on the specific goal of a booster dose: Is it prevent all illness, or just severe disease?
Another thorny question that the CDC advisors will continue to discuss Thursday is what people who were originally vaccinated with Moderna or Johnson & Johnson should do, as Wednesday's emergency use authorization only included people who received Pfizer's vaccine.
"That's going to leave half of people immunized being told that they’re at risk now or waning immunity and hospitalization and unable to get a booster dose. That’s a big public health panic that we would like to avoid,” said ACIP member Dr. Sarah Long, a professor of pediatrics at Drexel University College of Medicine in Philadelphia.
Moderna has submitted data on its booster dose to the FDA, and Johnson & Johnson is working on doing the same.
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Erika Edwards is a health and medical news writer and reporter for NBC News and "TODAY."
Sara G. Miller is the health editor for NBC News, Health & Medical Unit.