Pfizer Inc. and its German partner BioNTech said on Wednesday that they had started a mid-stage study of a modified COVID-19 vaccine that targets both the original and 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 percent of the circulating coronavirus variants in the United States for the week ending July 23.

The U.S. Food and Drug Administration (FDA) 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.

The BA.5 subvariant of Omicron was estimated to make up 81.9 percent of the circulating coronavirus variants in the United States for the week ending on July 23, the U.S. Centers for Disease Control and Prevention (CDC) said on Tuesday.

This was higher than the 75.9 percent prevalence estimated in the preceding week.

BA.5 has been driving a surge of new infections globally and has shown to be particularly good at evading the immune protection afforded either by vaccination or prior infection.

Omicron subvariant BA.4 was estimated to make up 12.9 percent of the circulating variants in the United States, the data showed.

The U.S. FDA has asked vaccine manufacturers to target the two currently dominant subvariants for a potential fall season booster dose.

U.S. health officials are also urging people aged 50 or older to get a booster shot, adding that doing so would not prevent them from getting another "bivalent" booster designed to fend off Omicron more specifically later this year.