The United States on Wednesday authorized the use of boosters of Pfizer's Covid-19 vaccine for people aged over 65, as well as adults at high risk of severe disease and those in high-exposure jobs.
The announcement means a significant part of the population -- amounting to tens of millions of Americans -- are now eligible for a third shot six months after their second.
"Today's action demonstrates that science and the currently available data continue to guide the FDA's decision-making for COVID-19 vaccines during this pandemic," said Janet Woodcock, acting head of the Food and Drug Administration, in a statement.
The decision was expected and came after an independent expert panel convened by the regulatory agency last week voted in favor of recommending the move.
The panel, however, rejected an initial plan by the White House to fully approve Pfizer boosters to everyone aged 16 and over, in what amounted to a rare rebuke of President Joe Biden's administration.
The group of vaccinologists, infectious disease specialists and epidemiologists concluded that the benefit-risk balance differed for younger people, especially young males who are more susceptible to myocarditis.
- More boosters debated -
Pfizer Covid-19 boosters are currently being debated by a separate body of experts convened by the Centers for Disease Control and Prevention (CDC), which may recommend further specifics about recipients.
For example, if obesity is considered as putting a person "at high risk of severe Covid," that definition would cover more than 42 percent of the US population aged over 20.
The CDC may also have to define which workplaces and other settings might lead to "frequent institutional or occupational exposure to SARS-CoV-2."
For its part, the FDA indicated this would cover "health care workers, teachers and day care staff, grocery workers and those in homeless shelters or prisons, among others."
The FDA's emergency use authorization (EUA) applies to those aged 18 and up for the high risk of severe disease and high-exposure categories. It also only applies to Pfizer's vaccine.
Recipients of Moderna and Johnson & Johnson, the other US-authorized vaccines, will now await news for when they, too, might become eligible for another shot.
A number of studies have shown two doses of Pfizer or Moderna, or a single shot of J&J, continue to confer high protection against severe outcomes -- but this is slightly reduced for the elderly.
The World Health Organization has called for a moratorium on wealthy countries giving out boosters, while many countries -- especially those in Africa -- have barely begun their immunization campaigns.
The United States argues, however, it is possible to both help middle- and lower-income nations while also protecting its own vulnerable people.
On Wednesday, President Biden announced the United States would buy 500 million more Pfizer doses for the world, bringing its total contribution of the global supply to 1.1 billion.
Speaking to CNN on Sunday, top government scientist Anthony Fauci said that more research is needed to make a decision on whether a booster shot is warranted for the general public.
"As we said in the beginning, we would want to plan for the possibility of vaccinating all those who have gotten their initial vaccination with Pfizer," Fauci said. "And it was always pending the evaluation of all of the totality of the data from the United States, from Israel, and any bit of data that we could get by the advisory committee to the FDA."
Fauci added that the FDA panel decided against a third shoot for everybody aged 16 and over "in the proper deliberative process and they came up with a recommendation."