Millions of Americans whose immune systems have been undermined by disease or medical treatment, and who are already fully vaccinated against Covid with the Pfizer or Moderna vaccines, will be allowed to get a third dose, according to federal decisions made Thursday night and Friday.

The decisions by the Food and Drug Administration and the Advisory Committee on Immunization Practices, an outside advisory group to the Centers for Disease Control and Prevention, let US health policy catch up to research showing that transplant recipients and cancer patients are not fully protected by two shots—and also to patients who were already getting unapproved doses on their own. Effectively, it changes the authorized vaccine series for people with weak immune systems from two doses to three, with the third dose delivered 28 days or more after the second.

But it isn’t a blanket change. It covers only people with what the FDA and the ACIP call “moderate to severe immune compromise.” It allows the third shots only for those age 12 and older, because no vaccine has been authorized for pre-teens or kids. And it does not approve any extra shots for the immune-impaired people who are among the 12 million recipients of the single-dose Johnson & Johnson vaccine, because there isn’t yet data to predict their reactions.

Both the FDA and the committee were careful to not call the third doses “boosters,” hoping to draw a distinction between improving initial protection in people with low immunity, and renewing immunity in the rest of the population if vaccine effectiveness fades against future variants. But, globally, the move might still be controversial. The World Health Organization has begged rich nations to stop delivering extra doses until poor nations can obtain more shots.

The decision to add a dose for immune-compromised people has been coming for a while. The ACIP, which is a group of medical researchers and representatives of professional societies who help the CDC shape vaccine policy, examined evidence for the move last month. CDC director Rochelle Walensky flagged its imminent arrival Thursday afternoon in a White House Covid-19 response team briefing. “This action is about ensuring our most vulnerable—who may need an additional dose to enhance their biological responses to the vaccines—are better protected against Covid-19,” she said.

The FDA changed its guidance on the vaccines late Thursday night, amending the Emergency Use Authorizations for the Pfizer and Moderna formulas to include the three-dose series and setting the stage for the committee, which had already scheduled a discussion Friday morning. That meeting ended in a unanimous vote in favor of third doses.

But the immediate trigger for the change—alongside the surge in the extra-contagious Delta variant that is swamping the US—was studies from Canada and France that were published Wednesday and Thursday in the New England Journal of Medicine (though shared with the federal agencies beforehand). Both documented that small groups of transplant recipients who had received two vaccine doses had low levels of antibodies, but gained significant immune protection after a third dose.

“I really believe these results are definitive; I think it’s a slam dunk,” Atul Humar, a physician and director of transplantation at the University of Toronto and senior author on the Canadian trial, said Thursday before the FDA and CDC acted. That study, in 120 organ transplant patients, showed not only antibody improvements, but also higher T-cell responses in the third-dose recipients compared to participants who received a placebo. “I think the results are convincing enough that this will result in a change in regulatory guidance, recommending a third dose for these patients,” he said.