The Food and Drug Administration cleared a third dose of the Pfizer / BioNTech COVID-19 vaccine for people some vulnerable groups — the first booster in the United States’ vaccination efforts.

The agency signed off on boosters for people 65 years of age and older, those who are at high risk of severe disease, health care workers, and other people at high risk of exposure to COVID-19 at work.

The decision comes a month after the Biden administration announced that booster shots would be available to people in the US starting in September. At the time, scientists and public health experts criticized the administration for pushing boosters before there was clear evidence that they were necessary and before either the FDA or CDC signed off on their use. In early September, federal officials told the White House that they may have to modify or push back the booster plan while health agencies collected and reviewed data, The New York Times reported. 

The decision only covers the Pfizer / BioNTech vaccine shots, and there are no updated guidelines for the Moderna and Johnson & Johnson shots at this point. 

Over the past few weeks, experts debated whether or not third doses of the Pfizer / BioNTech and Moderna COVID-19 vaccines were necessary. Data from the US and Israel appears to show that protection against infection with the coronavirus drops off over time, though there are different estimates around exactly how steep that decline might be. The effect is more pronounced for older people. Most data shows that otherwise healthy people who get two shots of the vaccine are still protected against severe illness. 

The FDA’s advisory committee on vaccination on Friday voted against recommending boosters for people 16 years of age and older, citing limited evidence for boosters in younger age groups. Members were also uncomfortable with the lack of safety data on third doses in younger people. The committee recommended boosters for a narrower group of people: those who are over 65 or who might be at high risk of severe COVID-19. In a less formal poll, it also supported boosters for health care workers and people at risk of exposure at work.

Most data used to make the case for or against boosters comes from outside the US, from countries that have centralized health care systems that they can pull from to understand big-picture COVID-19 trends. The US doesn’t have that kind of infrastructure, so vaccine information is more piecemeal. 

Israel is offering third doses of the COVID-19 vaccine to everyone over the age of 12. A third dose has been available to people over 60 in Israel since the end of July. Pfizer / BioNTech said in data released this week that the protection against infection had fallen for that group, which was among the first to be vaccinated, but that a third shot bumped the protection against infection back to the 95 percent efficacy seen in the original trials testing the vaccines. The United Kingdom also announced a booster plan this week, and will give third shots to people over the age of 50. 

Experts are concerned that the US and other wealthy countries are pursuing third doses while vaccines are still scarce in most parts of the world. The World Health Organization has called for a moratorium on boosters until the end of the year, in order to prioritize doses for low income countries. 

An advisory committee for the Centers for Disease Control and Prevention meets Thursday, and is expected to discuss if it recommends the now-authorized third shot, and who it recommends them for. The committee would refine and define the groups that should qualify for a booster under the FDA’s decision.