WASHINGTON — U.S. health officials said Wednesday they have agreed to purchase another 105 million doses of Pfizer’s COVID-19 vaccine in anticipation of a fall booster campaign.

The $3.2 billion deal announced by the Biden administration comes as federal scientists consider how to update the vaccines to better protect Americans from the rapidly evolving virus. Federal officials said the purchase agreement includes the option to purchase a total of 300 million doses, including a mix of doses for both adults and children.

The first shots would be delivered by early fall, pending a decision by the Food and Drug Administration to authorize new versions of the shots. A decision is expected from the FDA in the coming days following a Tuesday meeting in which outside advisers recommended modifying the vaccines to better target the omicron variant.

The current vaccines retain strong protection against hospitalization and death, but their ability to block infection dropped markedly when omicron appeared.

It’s not yet clear who would be offered a tweaked booster — they might be urged only for older adults or those at high risk from the virus.