Combined with 105 million doses the government already agreed to buy from Pfizer Inc PFE.N and partner BioNTech SE 22UAy.DE for $3.2 billion, the latest deal puts the U.S. booster dose supply at about 171 million shots.

COVID-19 infections in the United States have been on the rise as the BA.4 and BA.5 subvariants of the Omicron variant have spread to account for more than 90% of U.S. infections, according the U.S. Centers for Disease Control and Prevention.

These subvariants have significant mutations from the earliest versions of Omicron and protection from vaccines wanes over time.

The U.S. Food and Drug Administration in June recommended that vaccine manufacturers target these subvariants for a potential fall season booster dose in a bivalent vaccine that also includes the original strain.

Moderna’s initial 66 million doses are for delivery this year. The government’s deals with Moderna and Pfizer could go up to 300 million doses from each company if it exercises all of its options and secures funding from the Congress.

“As we look to the fall and winter… we are ensuring Americans have the tools they need to stay safe and help keep our nation moving forward,” U.S. Department of Health and Human Services Secretary Xavier Becerra said in a statement.

The booster candidates need to secure FDA authorization and a recommendation by the U.S. Centers for Disease Control and Prevention, before they can be made available for wider use.

(Reporting by Mrinalika Roy in Bengaluru; Editing by Caroline Humer and Shailesh Kuber)