Why Omicron variants BA.4 and BA.5 are causing fresh U.S. outbreaks

More infectious than past strains, these subvariants can also more easily escape antibodies from vaccines and previous infections.

Two Omicron subvariants are now causing more than half of new coronavirus infections in the United States—and both are very good at dodging antibodies in people who have been vaccinated and boosted, as well as in people who had a previous COVID-19 infection.

First spotted by scientists in South Africa in January and February this year, the BA.4 and BA.5 subvariants became dominant in the U.S. in less than two months, according to this week’s estimates from the U.S. Centers for Disease Control and Prevention. For the week ending on June 25, BA.4 accounted for 15.7 percent of new cases, while BA.5 was responsible for 36.6 percent.

“BA.4/BA.5 certainly is more infectious compared to previous Omicron variants,” says Yunlong Richard Cao, an immunologist at the Biomedical Pioneering Innovation Center at Peking University in Beijing, China. Cao’s research shows that one of the most concerning traits of these variants is their ability to evade the immune system and break through herd immunity.

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