A C.D.C. panel will meet to decide who should get Pfizer boosters.

Scientific advisers to the Centers for Disease Control and Prevention are scheduled to meet on Wednesday to decide which Americans should get booster shots of the Pfizer-BioNTech coronavirus vaccine — and when.

The committee’s discussions are normally a scientific affair of little interest to most people. But this meeting is likely to be closely watched — the stakes are high. It follows a dramatic exchange at the Food and Drug Administration on Friday, when advisers overwhelmingly rejected a proposal to offer Pfizer booster shots for Americans over 16, but then voted unanimously in favor of third doses for some high-risk people and those older than 65.

The role of the C.D.C. advisers on Wednesday is to decide who belongs in those high-risk groups. Depending on their verdict, booster shots could be offered to most Americans — or only to a select few.

The decision may come as late as Thursday. But it is likely to be too late for the Biden administration, which had planned to begin offering third doses this week to most fully vaccinated adults in the United States.

At the meeting on Friday, senior scientists at Pfizer and the Israeli Ministry of Health presented data they said indicated waning immunity in people who received the Pfizer vaccine months earlier. Boosting immune defenses with a third shot has made a difference in Israel, they said, and could stem the tide of infections in the United States.

The F.D.A. advisers also evaluated data from the C.D.C. on the trajectory of the virus in the United States, as well as summaries from several studies on the effectiveness of the vaccines.

But after reviewing the evidence, the scientists on the committee concluded that while protection against infection may be waning, especially in older adults, the original two-shot regimen still offers excellent protection against severe illness and hospitalization in most people.

“It’s unclear that everyone needs to be boosted, other than a subset of the population that clearly would be at high risk for serious disease,” said Dr. Michael G. Kurilla, a committee member and official at the National Institutes of Health.

One key difference between Israel and the United States may explain why the two countries have had different experiences with the vaccine: The countries define severe illness differently.

In Israel, anyone with an accelerated respiratory rate and an oxygen level of below 94 percent is severely ill. By contrast, the C.D.C. reserves that category for people who are sick enough to be hospitalized, said Dr. Sara Oliver, a C.D.C. scientist who presented the American data.

Some committee experts said they also did not feel comfortable offering booster shots to young people who may not need them, when the risks of a third dose are unknown.

The vaccines have been tied to rare cases of myocarditis, inflammation of the heart, in younger people. The risk is very small, and studies have shown that Covid-19 is much more likely to cause the condition.

Last week brought other research in support of the F.D.A. committee’s decision, including one paper by an international team of scientists that analyzed dozens of studies and concluded that the world would be better served by using vaccine doses to protect the billions who remain unvaccinated.

The authors of that study included the top two vaccine experts at the F.D.A., who announced plans to leave the agency this fall. Their departure is in part in protest of the Biden administration’s push for boosters before federal scientists and regulators had reviewed the evidence.