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The US Centers for Disease Control and Prevention is recommending that millions of people who have gotten their two shots of the Pfizer coronavirus vaccine get booster shots. Here’s a quick briefing on who’s eligible and a refresher on how we got to this point:

Who’s eligible?

The CDC said in a statement that the following people should receive booster shots six months after they received their initial Pfizer doses:

— People 65 and older and residents in long-term care settings.

— People 50 to 64 with underlying medical conditions that make them more likely to get severely ill.


The statement also said the following people may get booster shots six months after they received their initial Pfizer doses.

— People 18 to 49 with underlying medical conditions, “based on their individual benefits and risks.”

— People 18 to 64 who are at increased risk for COVID-19 exposure and transmission because of their “occupational or institutional setting ... based on their individual benefits and risks.” The occupational or institutional settings weren’t specified in the statement. But federal officials on Friday said that would include health care workers, teachers, grocery store workers, and people who live in homeless shelters or prisons. A CDC spokeswoman also pointed to materials discussed in a CDC advisory panel meeting this past week that suggested a wide range of essential workers would be eligible.

A handy chart

If you’re a visual person, this might help. Kizzmekia S. Corbett, who joined the faculty of the Harvard T.H. Chan School of Public Health this year and who formerly, as a federal researcher, was instrumental in developing the Moderna vaccine, posted this flow chart on Twitter.

Some caveats

— The booster shot recommendations are only for adults. If you’re under 18, you’re not eligible, no matter what.


— The CDC recommendation only applies to people who got the Pfizer vaccine, not the two-shot Moderna vaccine or the one-shot Johnson & Johnson vaccine. Pfizer took the lead in the race to get approval for the boosters. The other companies did not submit data in time to get considered this time around. “We will address, with the same sense of urgency, recommendations for the Moderna and J&J vaccines as soon as those data are available,” Dr. Rochelle Walensky, the CDC director, said in the statement.

— Mixing and matching — getting a Pfizer boost after getting a Johnson & Johnson shot, for example — is not recommended. Federal officials say studies are still underway on that.

— One other caveat: States can come up with their own recommendations on who gets booster shots. Massachusetts Governor Charlie Baker’s office said this past week that it was waiting to see what the CDC recommended. The office didn’t immediately have a response Friday.

How did we get here?

It may have been difficult to follow all the headline-generating, breaking booster news. Here’s a quick chronological rundown that should help:

— The Biden administration announced Aug. 18 that it wanted to get Pfizer and Moderna booster shots to all Americans, and the plan for that to begin this past week. But officials said they would abide by the decisions of the US Food and Drug Administration, which would authorize the booster, and the CDC, which issues guidance on vaccine policy for clinicians and public health officials throughout the United States. Things didn’t go exactly as the administration planned.


— Earlier this month, an FDA advisory panel recommended the Pfizer booster be authorized for a smaller group than Biden wanted. The panel’s vote was followed this past week by approval by the FDA commissioner.

— On the heels of the FDA approval, a CDC advisory panel on Thursday recommended that the boosters be used for an even smaller group than the FDA had recommended.

But hours later, CDC head Walensky, in an unusual twist, expanded the category of who is eligible for boosters, saying she was aligning with the FDA, rather than her own panel. She restored the eligibility of those such as health care workers and teachers, who are at increased risk because of their “occupational or institutional setting.”

She said at a White House COVID-19 response team briefing Friday that the decision to include those groups had been “a scientific close call” and “in that situation, it was my call to make.”

President Biden told reporters Friday, “The decision of which booster shot to give, when to start the shot, and who will get them, is left to the scientists and the doctors. That’s what happened here.”

He described the day’s events as a “key step in protecting the vaccinated with booster shots, which our top government doctors believe provides the highest level of protection available to date.”


Material from Globe wire services was used in this report. Christina Prignano of the Globe staff also contributed.

Martin Finucane can be reached at martin.finucane@globe.com.