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Vaccine advisers to the US Food and Drug Administration voted unanimously Thursday to recommend emergency use authorization of a booster dose of Moderna’s Covid-19 vaccine.

The FDA’s Vaccines and Related Biological Products Advisory Committee agreed use of a booster dose would be safe and effective in some people six months out from their primary series. But some were clearly reluctant.

Moderna had asked for emergency use authorization for a half dose of its vaccine to be used as a booster for certain people.

All 19 members of the committee supported authorizing a 50-microgram booster dose – half the size of the 100-microgram doses used in the primary series of the two-dose vaccine – at least six months after the second dose, and only for certain groups: people age 65 and older; people ages 18 to 64 who are at high risk of severe Covid-19; and people ages 18 to 64 whose exposure to the coronavirus in their settings or jobs put them at risk for Covid-19 complications or severe illness.

Moderna’s EUA request mirrored the groups authorized to receive a booster dose of Pfizer’s mRNA vaccine. Third doses of the Moderna and Pfizer Covid-19 vaccines are already authorized for some immunocompromised people.

Members of the committee said they were not entirely convinced the data showed a booster was necessary or that it increased protection.

“The data are not perfect but these are extraordinary times and we have to work with imperfect data,” said Dr. Eric Rubin, editor-in-chief of the New England Journal of Medicine and a professor at the Harvard T.H. Chan School of Public Health.

“We can’t afford to have health care workers, even if not sick, be positive and infected and have to stay home from work because in parts of the country there’s a shortage of health care workers and there’s burnout everywhere,” said Dr. Stanley Perlman, a professor of pediatrics at the University of California San Diego.

The VRBPAC meeting will continue Friday, when members are expected to vote on boosters for the Johnson & Johnson vaccine, and will hear a presentation on mix-and-match vaccines.

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The FDA’s committee of independent advisers typically discusses and makes recommendations to the agency around vaccine authorizations and approvals, and the agency then makes the final decision about whether to OK a vaccine.

CDC’s Advisory Committee on Immunization Practices is already scheduled to discuss boosters on October 21.

After Thursday’s vote, members were asked to discuss whether they’d consider recommending widening that group to younger ages. Several members said they were not even fully comfortable with having recommended boosters as widely as they already had.

“I don’t agree with doing this down to 18 years of age at all,” said Dr. Paul Offit, director of the Vaccine Education Center and a pediatrics professor at Children’s Hospital of Philadelphia.

“I am impressed by the fact that we continue to have excellent protection against moderate to severe disease in this country through Delta and for all age groups. And I just think we continue to send wrong messages out there by using terms like breakthrough and by making people feel like they’re not protected unless they’ve gotten a third dose,” he added.

“I am uncomfortable with how we just sort of tripped down the line here with the concept of a universal booster dose.”

Dr. Michael Kurilla, director of the Division of Clinical Innovation at the National Center for Advancing Translation Sciences at the National Institutes of Health, agreed.

“I don’t see the need for a let-it-rip campaign for boosters,” Kurilla said.

Several members noted that the real need is to get unvaccinated people in the US vaccinated for the first time, not to get booster shots into people’s arms.

“I think we heard pretty loud and clearly that there was not much appetite for moving down the age range very significantly, if at all,” said Dr. Peter Marks, who heads the FDA’s vaccine arm, the Center for Biologics Evaluation and Research. Marks said the FDA was concerned about unexpected new waves of disease.

If the FDA gives emergency use authorization to Moderna boosters, vaccine advisers to the US Centers for Disease Control and Prevention will meet to discuss which groups to recommend them to. Typically, shots can be administered once the CDC director signs off on the recommendation.

CNN’s Jacqueline Howard contributed to this report.