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. But once the FDA decides on the recipe change, Pfizer and competitor Moderna will have to seek authorization for the appropriately updated doses, time for health authorities to settle on a fall strategy.

Wednesday's announcement came as Congress remained gridlocked over billions in funding requested by the Biden administration to purchase additional vaccines, tests and drugs to fight the pandemic. House and Senate lawmakers have been wrangling for weeks over how to resolve the stalemate. Funding for the latest Pfizer purchase comes from reallocated money from earlier COVID-19 relief packages, officials said.