WASHINGTON — The head of the Food and Drug Administration says his agency has told Pfizer that it “will rapidly work” to grant emergency use of its COVID-19 vaccine following a positive recommendation by government advisers.

The FDA decision will kickstart an unprecedented vaccination campaign needed to eventually defeat the virus. The FDA’s greenlight of the vaccine, co-developed with BioNtech, was practically assured after the positive vote by agency advisers a day earlier.

The FDA’s brief statement came less than an hour after President Donald Trump tweeted directly at FDA Commissioner Stephen Hahn, complaining that FDA “is still a big, old, slow turtle.”

FDA staff have repeatedly said they expect to issue a decision within days of Thursday’s meeting. Many FDA observers predict action by Saturday ahead of a Sunday meeting by the Centers for Disease Control and Prevention.

The panel of CDC advisers will vote on who should get priority for the initial shots. Federal officials plan to allocate the first 6.4 million doses of the vaccine to states based on their population.