WASHINGTON >> An influential federal advisory panel overwhelmingly rejected a plan today to give Pfizer COVID-19 booster shots to most Americans, but it endorsed the extra shots for those who are 65 or older or run a high risk of severe disease.

The twin votes represented a heavy blow to the Biden administration’s sweeping effort to shore up nearly all Americans’ protection amid the spread of the highly contagious delta variant.

The decision was made by a committee of outside experts who advise the Food and Drug Administration.

In a surprising turn, the panel first rejected, by a vote of 16-2, boosters for nearly everyone. Members cited a lack of safety data on extra doses and also raised doubts about the value of mass boosters, rather than ones targeted to specific groups.

Then, in an 18-0 vote, it endorsed the extra shot for select portions of the U.S. population — namely, those most at risk from the virus.

That would help salvage part of the White House’s campaign but would still be a huge step back from the huge plan proposed by administration a month ago to offer booster shots of both the Pfizer and Moderna vaccines to nearly all Americans eight months after they get their second dose.

Today’s vote was just the first step. The FDA itself is expected to make a decision on boosters in the next few days, though it usually follows the committee’s recommendations.

The offering of boosters is also subject to approval by the Centers for Disease Control and Prevention. A CDC advisory panel is expected to take up the question on Wednesday. The CDC has said it is considering boosters for older people, nursing home residents and front-line health care workers, rather than all adults.

Separate FDA and CDC decisions will be needed in order for people who received the Moderna or J&J shots to get boosters.

During several hours of vigorous debate today, members of the panel questioned the value of offering boosters to nearly everyone.

“I don’t think a booster dose is going to significantly contribute to controlling the pandemic,” said Dr. Cody Meissner of Tufts University. “And I think it’s important that the main message we transmit is that we’ve got to get everyone two doses.”

Dr. Amanda Cohn of the Centers for Disease Control and Prevention said: “At this moment it is clear that the unvaccinated are driving transmission in the United States.”

Scientists inside and outside the government have been divided in recent days over the need for boosters and who should get them, and the World Health Organization has strongly objected to rich nations giving a third round of shots when poor countries don’t have enough vaccine for their first.

While research suggests immunity levels in those who have been vaccinated wane over time and boosters can reverse that, the Pfizer vaccine is still highly protective against severe illness and death, even amid the spread of the highly contagious delta variant.

The surprise turn of events could reinforce recent criticism that the Biden administration got out ahead of the science in its push for boosters. President Joe Biden promised early on that his administration would “follow the science,” following disclosures of political meddling in the Trump administration’s coronavirus response.

The FDA panel’s overwhelming rejection came despite full-throated arguments about the need for boosters from both Pfizer and health officials from Israel, which began offering boosters to its citizens in July.

Sharon Alroy-Preis of Israel’s Ministry of Health said the booster dose improves protection tenfold against infection in people 60 and older.

“It’s like a fresh vaccine,” bringing protection back to original levels and helping Israel “dampen severe cases in the fourth wave,” she said.

And representatives for Pfizer argued that it is important to shore up immunity before protection against severe disease starts to erode. A company study of 44,000 people showed effectiveness against symptomatic COVID-19 was 96% two months after the second dose, but had dropped to 84% by around six months.

Both Pfizer and the Israeli representatives faced pushback from panelists. Several expressed skepticism about the relevance of Israel’s experience to the U.S. Another concern was whether third doses would exacerbate serious side effects.

Meissner said he is worried about extra doses for younger age groups given the risk of heart inflammation that has been seen in mostly younger men after a second dose. While the condition is very rare, he said, it is not clear if that risk would increase with another dose.

Pfizer pointed to Israeli data from nearly 3 million boosters to suggest side effect rates would be similar to that seen after second doses.

Dr. Paul Offit, a vaccine expert at Children’s Hospital of Philadelphia, said he was more likely to support approving a third dose for adults over 60 or 65 but “I really have trouble” supporting it for anyone down to age 16.

While an extra shot likely will at least temporarily decrease cases with mild or no symptoms, “the question becomes what will be the impact of that on the arc of the pandemic, which may not be all that much,” Offit said.

Biden’s top health advisers, including the heads of the FDA and CDC, first announced plans for widespread booster shots a month ago, targeting the week of Sept. 20 as an all-but-certain start date. But that was before FDA staff scientists had completed their own assessments of the data.

Earlier this week, two top FDA vaccine reviewers joined a group of international scientists in publishing an editorial rejecting the need for boosters in healthy people. The scientists said continuing studies show the shots are working well despite the delta variant.

Today, U.S. Surgeon General Dr. Vivek Murthy said that in announcing its booster plan, the Biden administration was not trying to pressure regulators to act but was instead trying to be transparent with the public and be prepared in the event that extra shots won approval.

“We have always said that this initial plan would be contingent on the FDA and the CDC’s independent evaluation,” Murthy said.

The Biden plan has also raised major ethical concerns about impoverished parts of the world still clamoring for vaccine. But the administration has argued that the plan is not an us-or-them choice, noting that the U.S. is supplying large quantities of vaccine to the rest of the globe.

The U.S. has already approved Pfizer and Moderna boosters for certain people with weakened immune systems, such as cancer patients and transplant recipients.

Some Americans, healthy or not, have managed to get boosters, in some cases simply by showing up and asking for a shot. And some health systems already are offering extra doses to high-risk people.