A US advisory panel has overwhelmingly rejected a plan to give Pfizer COVID-19 booster shots to most Americans, but it has endorsed the extra shots for those who are 65 or older or run a high risk of severe disease.
The twin votes represent a heavy blow to the Biden administration's sweeping effort, announced a month ago, to shore up nearly all Americans' protection amid the spread of the highly contagious Delta variant.
The non-binding recommendation on Friday - from an influential committee of outside experts who advise the Food and Drug Administration - is not the last word.
The FDA will consider the group's advice and make its own decision, probably within days. And the Centers for Disease Control and Prevention is set to weigh in next week.
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 extra shots for people 65 and older and those at risk of serious disease.
Panel members also agreed that health workers and others who run a high risk of being exposed to the virus on the job should get boosters, too.
That would help salvage part of the White House's campaign but would still be a huge step back from the far-reaching proposal to offer third shots of both the Pfizer and Moderna vaccines to Americans eight months after they get their second dose.
The White House sought to frame the action as progress.
"Today was an important step forward in providing better protection to Americans from COVID-19," White House spokesman Kevin Munoz said.
"We stand ready to provide booster shots to eligible Americans once the process concludes at the end of next week."
The CDC has said it is considering boosters for older people, nursing home residents and front-line health care workers, rather than all adults.
The FDA and CDC will most likely decide at some later point whether people who received the Moderna or Johnson & Johnson shots should get boosters.
Scientists have been divided over the need for boosters and who should get them, and the World Health Organisation has strongly objected to rich nations giving a third round of shots when poor countries don't have enough vaccine for their first.
Australian Associated Press