Pfizer Inc and its German partner BioNTech SE said on Wednesday they had started a mid-stage study of a modified COVID-19 vaccine which targets both the original as well as the BA.2 omicron subvariant.

Pfizer said the vaccine is in an initial proof-of-concept study to gather more data.

Together, the BA.4 and BA.5 subvariants are estimated to make up nearly 95% of the circulating coronavirus variants in the United States for the week ended July 23.

U.S. Food and Drug Administration last month asked vaccine manufacturers to target BA.4 and BA.5, the two currently dominant omicron subvariants, for a potential fall season booster dose.