A decision about boosters from the FDA is expected by next week, and a Centers for Disease Control and Prevention advisory committee is slated to meet Wednesday and Thursday to recommend how a third shot should be used. The FDA advisory committee, following Pfizer’s lead, recommended that the third shot be given at least six months after the second.

Friday’s protracted online meeting, the most important FDA advisory committee meeting since the vaccines were first authorized, gave the Biden administration and Pfizer some, but not all, of what they wanted. Boosters will be on the way into many millions of arms — with the exact number depending on how the FDA and the CDC decide who meets the criteria for being at high risk of serious illness.

The consideration of booster shots comes as the United States endures a fourth wave of covid-19 infections, with hospitals in some corners of the nation confronting the long-feared prospect of rationing care and having to decide which patients receive access to treatments and medical equipment. And the debate has sparked criticism from some officials in the global health community who argue that the U.S. discussion of boosters betrays selfishness, as many in the world do not have access to a first dose of vaccine.

The recommendation to target shots primarily to older adults is far narrower than what the companies and top officials in the Biden administration had sought: a blanket approval to boost anyone 16 and older. The panel voted resoundingly against a broadly available booster. Many committee members said they felt uncomfortable about whether the benefits outweighed the risks to younger adults, citing the lack of robust safety data.

The meeting then took an unusual turn. The voting question was reformulated, to ask if members of the committee thought a booster would be safe and effective for a narrower slice of the population. The panel voted unanimously yes.

The vote and the variety of views on display during hours of debate Friday, even among experts, could complicate the Biden administration’s effort to extend boosters to all adults beginning next week — and exacerbate public confusion.

“Today was an important step forward in providing better protection to Americans from COVID-19,” White House spokesman Kevin Munoz said in a statement. “We stand ready to provide booster shots to eligible Americans once the process concludes at the end of next week.”

The committee’s deliberations did not address many of the questions circulating among the public, including the roughly 81 million people who have received shots made by Moderna and Johnson & Johnson and whose eligibility for a booster may not be decided for weeks.

The all-day meeting revealed an array of opinions among America’s top medical experts on whether boosters are necessary and, if so, when and for whom.

“It would be great to wait until we have all the data about safety,” said Jay M. Portnoy, a professor of pediatrics who works at Children’s Mercy Hospital in Kansas City, Mo. “I’d rather not get the covid disease. I’d rather get the third dose of the vaccine.”

After Pfizer’s initial request for a booster broadly available to the general population was voted down, the panelists had a freewheeling discussion about what the age limit should be for getting an additional shot.

Paul A. Offit, a vaccine expert at the Children’s Hospital of Philadelphia, said he favored boosters for people age 65 and up, after which Eric J. Rubin, editor in chief of the New England Journal of Medicine, said, “I’m 63, so I like the 60 age instead of the 65 age.” Portnoy said he planned to get his dose next week — and then said he would get one the very next day.

Those divisions extended even within the FDA.

“We know that there may be differing opinions about the interpretation of the data regarding the potential need for additional doses, and we strongly encourage all the different viewpoints to be voiced and discussed regarding the data, which is complex and evolving,” Marks said at the introduction of the meeting.

Marks, in addressing the panel, showed his hand about his views without explicitly spelling them out. He noted that many vaccines require additional doses six months down the line, so it “should not be a surprise” if the coronavirus vaccines need another dose as well.

He also said many other vaccines are used not just to prevent severe illness and hospitalizations but also mild cases. And he mentioned the importance of preventing the spread of the virus to vulnerable populations such as younger children, for whom a vaccine has not been approved.

Two career FDA scientists who co-authored a highly unusual paper in the medical journal Lancet this month, arguing that boosters were not needed in the general population, also asked questions that betrayed their inclinations. Both have announced that they will soon step down from the agency.

Phil Krause, one of those FDA officials, asked a pointed question about some of the data Pfizer is using to support the case for boosters.

“Part of this, of course, is the difficulty of looking at this kind of data, without having the chance for FDA to review it or allowing for this kind of data to go through the peer-review process,” Krause said, adding that Pfizer was a co-sponsor of the study.

Marion Gruber, the other FDA official, raised the issue of safety earlier in the meeting, pointing out that the risk of heart inflammation after vaccination, though rare, is highest among younger males, who may have different risks and benefits from the vaccine than do older adults.

Some Americans have already found unofficial ways to get additional vaccine doses, and that number is only expected to increase, experts said. A third dose is already recommended for people with compromised immune systems.

Advisory panel members heard presentations from the FDA, the CDC and Pfizer. The data was often conflicting.

CDC scientist Sara Oliver presented data showing that while protection against milder infections has waned over time, protection against severe disease remains robust, even among older adults. She showed unpublished data that found that through July, adults 75 and older were 88 percent protected against hospitalization.

But scientists from Israel showed that in their highly vaccinated population, protective immunity from vaccination had clearly waned as the country confronted the delta surge this summer. That resulted in a huge uptick in infections, they found — and in severe cases, even among people who were fully vaccinated.

The government there decided to implement a booster campaign at the end of July, beginning with people older than 60, and presented data suggesting that it may have helped save hospitals from being overwhelmed.

Cases “were doubling every 10 days, and we got to places with thousands of cases, doubling every 10 days. It was scary,” said Sharon Alroy-Preis, director of public health services for Israel’s Ministry of Health. “If we had not started boosters at the end of July, we would have come to the capacity of Israeli hospitalization capabilities — and gone beyond it.”

One of the studies from Israel, published Wednesday in the New England Journal of Medicine, showed that people 60 and older who were given a booster shot had an 11-fold lower risk of contracting an infection than people who did not receive the additional shot. Research data from Israel’s Ministry of Health, posted online Wednesday by the FDA in advance of its presentation at Friday’s advisory committee meeting, also showed that the booster campaign dramatically lowered the rate of severe cases among people 60 and older.

Pfizer officials have depended heavily on the data from Israel in making their case that boosters are necessary.

“The Israeli experience could portend the U.S. covid-19 future — and soon,” said William C. Gruber, senior vice president of vaccine clinical research and development at Pfizer.

The booster issue has been swamped with scientific and political controversies in recent weeks. Senior Biden administration officials, worried about data showing a waning of vaccine efficacy, announced in mid-August that boosters would be available the week of Sept. 20, pending FDA and CDC sign-offs.

Many scientists were outraged by the decision to make a political announcement ahead of the scientific confirmation that boosters were safe, effective and necessary. Those scientific agencies subsequently told the White House that only the Pfizer-BioNTech product, whose data was the first to be filed at the agency, could be cleared by then.

Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases, has spoken forcefully in favor of boosters. In an interview Thursday with The Washington Post, he said the Israeli data suggests that a third shot might prevent viral transmission — something that could help curb the pandemic.

Fauci said some scientists seem to believe “it is okay” for vaccinated people to get infected as long as they experience only mild or moderate symptoms and don’t end up in the hospital. But, he said, “as a clinical person who sees a lot of patients, that isn’t okay,” adding that even mild infections can result in missed work, disruptions of family life and potential cases of long covid, with its debilitating effects.

Yasmeen Abutaleb and Lena H. Sun contributed to this report.