Millions of COVID vaccines have made their way into people's arms, but might some of those vaccinated folks need boosters to shore up their protection? That question is at least partially settled.

The US Food and Drug Administration (FDA) on September 22 granted emergency use of a single booster dose of the Pfizer-BioNTech COVID vaccine for certain individuals. Fewer than 24 hours later, an advisory panel to the Centers for Disease Control and Prevention (CDC) provided guidance on the administration of those doses.

And now CDC Director Rochelle Walensky, MD, has weighed in with the agency's decision.

So who, exactly, may be eligible for a booster? Here's the latest from the FDA and the CDC.

What is the FDA's recommendation?

The FDA amended the "emergency use authorization" (EUA) for Pfizer's Comirnaty vaccine. which is intended to prevent COVID-19. Under the EUA, a booster dose may be given at least six months after someone has completed the primary two-dose series.

The FDA specifically green-lighted boosters for three groups of people:

Individuals ages 65 and older.
Individuals ages 18 to 64 at "high risk of severe COVID-19."
Individuals 18 to 64 at high risk of serious COVID complications, including severe COVID, due to their "frequent institutional or occupational" exposure to SARS-CoV-2.

While Pfizer's vaccine can indeed be given to 16- and 17-year-olds, the booster recommendation doesn't apply to those adolescents.

Who should get the booster shot?

In the US, the CDC is responsible for setting vaccination schedules. It does so based on recommendations by the Advisory Committee on Immunization Practices (ACIP), an advisory body whose members weigh the evidence on a variety of factors, including the safety and effectiveness of vaccines given to people at specific ages.

Essentially, it's up the the CDC to recommend how the expanded EUA for Pfizer's COVID vaccine gets implemented. On September 23, the ACIP took four separate votes on a Pfizer booster. Later in the day, Dr. Walensky issued a statement agreeing with ACIP on three specific groups of people who should either get a booster or consider doing so, and siding with the FDA on a fourth group exposed to COVID where they work or live.

CDC now recommends:

Boosters for older adults. People 65 and older and long-term care residents should receive a booster at least six months after their initial vaccination. (ACIP voted unanimously in favor of boosting this group.)

Boosters for 50- to 64-year-olds with underlying medical conditions. Again, if you're one of these folks, CDC recommends a getting booster dose at least six months after completing the two-dose primary series. (Only two ACIP members voted against the recommendation.)

A possible booster dose for adults 18 to 64 with underlying health conditions. Specifically, CDC said these individuals "may" receive a booster at least six after vaccination based on "individual benefits and risks." That's in line with ACIP's recommendation.

A possible booster for people 18 to 64 at increased risk for COVID exposures and transmission. CDC sided with the FDA on this point, concluding that people at greater risk due to an "occupational or institutional setting" (in other words, where they work or live) "may" get a booster six months after completing their vaccination. (ACIP members voted 9-6 against that proposed use. )

"I believe we can best serve the nation's public health needs by providing booster doses for the elderly, those in long-term care facilities, people with underlying conditions, and for adults at high risk of disease from occupational and institutional exposures to COVID-19," Dr. Walensky said in a prepared statement.

What about the unvaccinated?

ACIP's meeting ran one-and-a-half hours longer than anticipated, with plenty of discussion about the potential for these recommendation to undermine confidence in the vaccines, which continue to provide strong protection against severe disease, especially in people under 65.

"I worry that we're getting distracted by the question of boosters for Pfizer when we have bigger and more important things to do in this pandemic," commented ACIP member Helen Keipp Talbot, MD, an infectious disease specialist at Vanderbilt University. "Hospitals are full because people are unvaccinated," she noted. "Honestly, we could give boosters to people, but that's not really the answer to this pandemic."

What about people who got Moderna or Johnson & Johnson?

You may have to wait a bit longer for decisions on booster doses if you got a COVID vaccine from another manufacturer. But Dr. Walensky assures the public that CDC will address, "with the same sense of urgency," recommendations for boosters of other COVID vaccines "as soon as those data are available."

Moderna announced on September 1 that it has submitted initial data to the FDA for a third dose given six months after completing the first two doses.

More recently, Johnson & Johnson said it has new data showing that a booster increases protection against COVID-19. A second shot given two months after the first dose provides 94% protection, the company said, while a booster given six months after the single-dose vaccine resulted in a 12-fold increase in antibody protection four weeks after the booster. To date, the company has not announced that it is seeking FDA approval for a booster dose.