An influential panel of advisers to the Centers for the Disease Control and Prevention convened on Wednesday to debate which Americans should get COVID-19 booster shots and when — a question that has proved more contentious than the Biden administration apparently expected.

The meeting came days after a different advisory group — this one serving the Food and Drug Administration — overwhelmingly rejected a sweeping White House plan to dispense third shots to nearly everyone. Instead, that panel endorsed booster doses of the Pfizer vaccine only for senior citizens and those at high risk from the virus.

While the COVID-19 vaccines continue to offer strong protection against severe illness, hospitalization and death, immunity against milder infection seems to be dropping months after vaccination.

Last week’s FDA advisory panel decision was only the first hurdle as the government sets its booster policy. The FDA itself still has to decide whether it agrees with its advisers’ recommendation and will authorize Pfizer boosters.

If it does, the CDC then must recommend who should get the extra shots after hearing from its own Advisory Committee on Immunization Practices, whose meeting was scheduled to extend into Thursday.

The priority still is to vaccinate the unvaccinated, who the CDC says account for the vast majority of COVID-19 cases, now soaring to levels not seen since last winter. About 182 million Americans are fully vaccinated, or nearly 55% of the population.

“I want to highlight that in September of 2021 in the United States, deaths from COVID-19 are largely vaccine-preventable with the primary series of any of the three vaccines available,” said Dr. Matthew Daley, a researcher at Kaiser Permanente Colorado and CDC adviser who opened Wednesday’s deliberations.

And the public must understand that no matter how good a COVID-19 vaccine, when it comes to milder infections, “it is unlikely that we will prevent everything,” said Dr. Helen Keipp Talbot of Vanderbilt University.

Many experts are torn about the need for boosters because they see the COVID-19 vaccines working as expected. It is normal for virus-blocking antibodies to be highest right after vaccination and then wane over the following months.

“We don’t care if antibodies wane. You care what is the minimum” needed for protection, said Dr. Sarah Long of Drexel University.

No one yet knows the antibody level threshold below which someone’s risk for infection suddenly jumps. Even then, the body has back-up defenses.

Antibody production and even those back-up defenses don’t form as robustly in older people. But it’s impossible to pinpoint the age at which that becomes a problem, CDC microbiologist Natalie Thornburg told the committee.

Ultimately the committee must decide who is considered at high enough risk for an extra dose. CDC officials presented data from several U.S. studies, saying there is growing evidence of a decline in the effectiveness of both the Pfizer and Moderna vaccines in preventing new COVID-19 infections in some groups, most notably people 65 and older and health care workers who got shots early in the vaccination campaign.

There’s also a hint that at age 75, there may be some decline in protection against hospitalization. But the CDC said there is little information on waning immunity in younger people with chronic medical problems.

Another question was how many months after the second shot the booster should be given. Scientists have talked about six months or eight months.

And if a Pfizer booster is cleared, what if someone who originally got Moderna or Johnson & Johnson shots wants one? A major U.S. study on mixing-and-matching booster doses isn’t yet complete, and FDA’s Dr. Doran Fink said a fact sheet for recipients would probably note there is no data on whether that is safe or effective.

As for booster safety, serious side effects are exceedingly rare with the first two doses. And Pfizer pointed to 2.8 million booster doses given in Israel, mostly to people 60 and older, with fewer reports of annoying side effects like pain or fever with the third dose than with the earlier shots. There was one report of a rare risk, heart inflammation, that is sometimes seen in younger men.

In the U.S., more than 24,000 people who have volunteered for a CDC vaccine safety tracking system have reported getting an extra dose, and likewise have reported no red flags.

The government will decide at a later date whether to allow extra doses of Moderna or J&J vaccines.