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We’ve seen a steep decline in the number of COVID-19 cases since winter’s record-breaking surge, but the variant responsible for that swell hasn’t made as swift a retreat. In fact, omicron has given rise to a number of subvariants — think of them like branches in the omicron family tree — that continue to drive new infections in the U.S. and other areas of the world.
BA.2, sometimes called “stealth omicron,” is responsible for about 62 percent of new COVID-19 cases in the U.S. And its descendant, BA.2.12.1, is behind nearly 37 percent of new infections, the latest data from the Centers for Disease Control and Prevention (CDC) shows. Global health officials are also keeping an eye on omicron subvariants BA.4 and BA.5, which have been reported in South Africa and some countries in Europe.
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Omicron’s subvariants appear more contagious
It’s not unusual for coronavirus variants to produce sublineages, says Egon Ozer, M.D., an assistant professor at Northwestern’s Feinberg School of Medicine and director of the Center for Pathogen Genomics and Microbial Evolution at the Havey Institute for Global Health. In fact, the delta variant, which dominated the summer and fall of 2021, had several.
The difference is, “they tended to kind of coexist,” Ozer says about the subvariants that made up delta’s family tree. “They didn’t seem like they were necessarily competing with each other.”
With omicron, however, the latest offshoots seem to have a leg up on their parent variant when it comes to transmissibility. The BA.2 subvariant edged out the original omicron strain (also known as BA.1) for the dominant spot after the winter surge, essentially rendering it obsolete. And BA.2.12.1 “might have a transmission advantage of about 25 percent over the BA.2 subvariant,” CDC Director Rochelle Walensky said in an April 26 news briefing.