October 20, 2021 7:13 PM EDT

The Food and Drug Administration (FDA) today followed the advice of its advisory committee and recommended booster shots of COVID-19 vaccines from Moderna and Johnson&Johnson-Janssen. The agency also authorized mixing or matching booster doses, meaning that people can either get another dose of the same vaccine they originally received, or get a booster with a different vaccine. More than 150 million doses of the Moderna vaccine and 15 million of the J&J-Janssen vaccine have been administered in the U.S.

In a briefing discussing the agency’s decision, acting FDA director Dr. Janet Woodcock said the primary reason behind recommending boosters is growing evidence that immunity provided by the original COVID-19 immunizations is starting to wane. “The availability of authorized boosters will be important for continued protection against COVID-19 disease,” she said. “The actions taken today help address waning immunity.”

The decision means that anyone who has been vaccinated against COVID-19—with one of the three available vaccines in the U.S.—could eventually get a booster shot. Until today’s decision, only people who were initially vaccinated with Pfizer-BioNTech’s shot, or those with weakened immune systems, could get an extra dose. The FDA clarified that for now, people who are eligible for a booster include those already vaccinated with Pfizer-BioNTech or Moderna who are over age 65, or anyone aged 18 to 64 who has an underlying health condition or works or lives in a setting that puts them at higher risk of exposure to COVID-19. That includes teachers, public transportation employees and essential health-care and front-line workers, as well as those living in homeless shelters or prisons. Anyone previously vaccinated with J&J-Janssen’s vaccine is eligible for a booster, however, given the lower initial efficacy of that vaccine compared to the other two.

On Sept. 22, the FDA authorized booster shots for people immunized with the Pfizer-BioNTech shot, and the Centers for Disease Control and Prevention (CDC) supported that decision, with subtle caveats. While the CDC recommends Pfizer-BioNTech boosters for people over age 65 and those with health conditions that might put them at higher risk of developing severe COVID-19, the agency stopped short of recommending them for younger people. CDC director Dr. Rochelle Walensky instead decided to allow younger people in higher risk jobs or settings to get an additional dose if they so choose.

The FDA’s recommendation on the Moderna and Johnson&Johnson-Janssen boosters follows the guidance the agency made for the Pfizer-BioNTech booster, with a few exceptions. The Pfizer-BioNTech booster is another dose of the same shot that people already received twice, while the Moderna booster is half the dose of the original shot. Moderna’s scientists showed that the lower dose produced a strong enough immune response and generated fewer potential side effects; they noted that the half dose would also double the number of people who could get a booster shot.

J&J-Janssen’s vaccine is the only one that requires a single dose. Unlike both the Pfizer-BioNTech and Moderna boosters, which are recommended at least six months after the initial vaccination, J&J-Janssen’s booster is recommended starting two months after the first dose. It is also recommended for anyone who received the shot, without any restrictions, because of its lower efficacy initially in protecting against COVID-19 compared to the Pfizer-BioNTech and Moderna vaccines.

Woodcock and Dr. Peter Marks, director of the Center for Biologics Evaluation and Research, acknowledged that rolling out boosters, with their different dosing schemes and target populations, will be complicated. That’s why the agency authorized the mixing and matching of booster doses, to allow providers the flexibility of boosting people with any COVID-19 vaccine if people can’t recall which one they were immunized with, or if it’s not practical for mobile vaccination teams or pharmacies or hospitals to carry all three vaccines.

It’s now up to the CDC’s Advisory Committee on Immunization Practices, which meets on Oct. 21, to iron out the details of how the boosters will be administered. Once the committee reviews and discusses the data and the FDA’s decision, Walensky will issue a final recommendation. The White House has said that the federal government, which will pay for booster shots as it did for the original COVID-19 vaccinations, is ready to start shipping doses so eligible people will have access soon.

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