WASHINGTON (AP) — The U.S. government will purchase another 105 million doses of Pfizer’s COVID-19 vaccine in anticipation of a fall booster campaign. The $3.2 billion agreement comes as federal scientists weigh whether and how to update the vaccines to better protect Americans from the rapidly evolving virus. Federal officials say 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 revamp and authorize new versions of the currently available vaccines.