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.