A highly contagious sublineage of the BA.2 omicron subvariant is now the nation’s dominant strain, according to the CDC’s latest variant proportion estimates. 

The sublineage, BA.2.12.1, accounted for 57.9 percent of all U.S. COVID-19 cases in the week ending May 21, CDC data shows. BA.2, which became the nation’s dominant strain in mid-March, now accounts for an estimated 39.1 percent of all cases.

BA.2.12.1 is estimated to have a 25 percent growth advantage over BA.2, which is already more transmissible than the original omicron strain. The newer omicron sublineage has been gaining traction in the U.S. over the last month. In the week ending April 23, BA.2.12.1 accounted for just 24.1 percent of U.S. COVID-19 cases. 

Health officials are also monitoring another omicron subvariant — BA.1.1.529 — which currently accounts for an estimated 2.8 percent of cases.  

“Epidemiologically, it doesn’t appear as if we’re seeing more severe disease in places that are having more cases,” CDC Director Rochelle Walensky said of the sublineages during an April 26 news conference. “So we are not anticipating more severe disease from some of these subvariants, but we are actively studying it.”