Both vaccines also include the original version of the virus targeted by all the previous Covid shots as the country prepares for another vaccination campaign in the fall season.

As a single dose, Moderna’s MRNA.Ovaccine is authorised for those aged 18 and above, while Pfizer’s bivalent candidate is for those aged 12 and above.

The US government has purchased 175 million doses of the booster shots in an effort to stave off the worst effects of a potential surge in new infections as schools reconvene and people spend more time indoors as the weather grows colder.

The FDA in June asked vaccine makers to tailor shots to the two subvariants responsible for the most recent surge in infections worldwide. The BA.5 subvariant currently accounts for more than 88% of US infections.

The US Centers for Disease Control and Prevention’s vaccine advisory panel is scheduled to meet on Thursday to make recommendations for use of the redesigned shots that director Rochelle Walensky will take into consideration before making a final decision.