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

Covid-19 infections in the US have been on the rise as the BA.4 and BA.5 sub-variants of the Omicron variant have spread to account for more than 90% of US infections, according the US Centers for Disease Control and Prevention.

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

The US Food and Drug Administration in June recommended that vaccine manufacturers target these sub-variants 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,” US department of health and human services secretary Xavier Becerra said in a statement.

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