People over age 65 and adults at high risk of severe COVID-19 can receive a booster dose of the Pfizer Inc.-BioNTech SE vaccine, the Food and Drug Administration said, opening a new and more controversial phase of the U.S. immunization campaign.

The emergency-use authorization also allows boosters for people 18 and older whose occupational exposure to the SARS-CoV-2 virus puts them at elevated risk of serious complications. Booster doses can be given any time at least six months after a person received their second shot, the FDA said in a statement.

Acting FDA Commissioner Janet Woodcock said the authorization would allow people such as teachers and day-care staff, as well as health care and grocery workers, to get the third shot.

The authorization applies only to the Pfizer-BioNTech shot, the agency said.

The emergency clearance is in line with a recommendation last week from FDA advisers but narrower than the full approval that Pfizer and its German partner had sought. It means that the Biden administration will have to move more slowly on its planned wider rollout of boosters proposed last month. Third doses were previously authorized for certain people with compromised immune systems.

“We believe boosters have an important role to play in addressing the continued threat of this disease, alongside efforts to increase global access and uptake among the unvaccinated,” Pfizer Chief Executive Officer Albert Bourla said in a statement. “Today’s FDA action is an important step in helping the most vulnerable among us remain protected from COVID-19.”

While less broad than some had hoped, the authorization leaves the door open for the FDA to consider broader use of boosters, including in younger adults, as more data on their safety and efficacy become available. And it assures that many of the older Americans who were first in line for the initial shots will be among the earliest to get additional protection.

A surge in virus infections caused by the delta variant has coincided with fears that vaccine potency will fade as the weather turns colder.

Scientists have been divided on the need for boosters. Real-world studies suggested that the efficacy of the Pfizer vaccine diminished somewhat this summer, especially in preventing mild breakthrough cases, though it is hard to distinguish waning immunity from the effects of the highly infectious delta variant.

But other data has suggested that the shot provides lasting protection against severe disease leading to hospitalization and death, and some researchers have said that boosters for most people could be put off for some time.

Pfizer shares rose 0.8% in after-hours trading on Wednesday, while shares of BioNTech gained 0.5%.

The FDA said it authorized the booster in part because of an analysis from Pfizer showing that people who got the vaccine earlier in a clinical trial showed “a modest decrease in the efficacy” in July and August, compared with those who had received the vaccines later.

In addition, the agency considered real-world effectiveness data on the vaccine’s efficacy from the U.K., U.S. and Israel.

Safety of the booster doses was studied in over 300 people, most of them ages 18 to 55. The most common side effects were pain and swelling at the injection site, fatigue, headache and muscle pain. Swollen lymph nodes in the armpit were more common after the booster dose than the first two doses, the agency said.

The Centers for Disease Control and Prevention’s Advisory Committee on Immunization Practices is expected to meet Thursday to make its own recommendations about who should receive the additional dose.

The panel, made up of outside vaccine experts, met Wednesday for a general review of vaccine efficacy over time and booster-shot science.

Some members raised the question of whether people who had received a Moderna Inc. or Johnson & Johnson vaccine for their primary immunization should receive the Pfizer booster, or whether they should wait to match with a supplemental dose of the shot they first received.

FDA vaccine official Doran Fink told the panel on Wednesday that the agency is “working as rapidly as possible” to review Moderna’s booster submission.

Since the FDA said Aug. 13 that people with weakened immune systems can receive a third shot, more than 2 million Americans have gotten one, according to CDC data.

The clearance for now leaves booster shots off the table for younger recipients of the Pfizer shot, which was approved for ages 16 and up earlier this year and can be given to adolescents from 12 to 15 under an emergency authorization.

There has been concern among some scientists, including members of the FDA’s vaccine advisory panel, about the risk of heart-related side effects from the vaccine, particularly in younger males.

Use of a third shot to protect vulnerable people was backed by recent evidence from other countries. An Israeli study in the New England Journal of Medicine showed a third dose of the Pfizer vaccine can dramatically cut rates of COVID-19-related illness in people age 60 and older.

Additionally, a Pfizer study suggested that the efficacy of its first two shots waned in a matter of months, and that booster doses are an effective way to combat the spread of the virus and new variants.

Some critics have said that booster shots should wait until immunizations are more widespread globally. World Health Organization Director-General Tedros Adhanom Ghebreyesus has argued that uneven vaccine distribution will be the world’s biggest obstacle to ending the pandemic and recovering.

Prior to last week’s meeting of the vaccine advisory panel, top scientists and two FDA officials, in a review published in The Lancet, questioned the scientific support for a third shot, saying available doses would be better used to immunize the unvaccinated.

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