The Centers for Disease Control and Prevention estimates the BA.2 sub-lineage of the omicron variant is now responsible for a quarter of new COVID-19 infections in the United States.

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The number of cases linked to BA.2 jumped from 10% last week to 25% this week.

According to the CDC’s “Nowcast,” the highest prevalence of BA.2 infections is being seen in the Northeast, with 39% of circulating viruses in the New York and New Jersey area being BA.2.

“Although the proportion of infections with BA.2 is increasing in the U.S., COVID-19 cases are now declining, so it is likely that absolute numbers of BA.2 infections are not increasing as quickly as they might seem from just looking at the proportion that are BA.2,” Dr. Deborah Dowell, the CDC’s chief medical officer for the agency’s COVID-19 response, said over the weekend.

A study from Japan suggested that BA.2 is not only more transmissible than the original omicron strain, BA.1, but may cause more severe disease.

China announced in the past week that it would be locking down Shanghai, with 25 million residents, and Shenzhen, with 11 million residents, because of the sub-lineage of the omicron variant.

The lockdowns include closing schools and nonessential businesses.

China has recorded more than 15,000 cases since the start of March, The New York Times reported.

“The epidemic in some areas is surging rapidly, and the risk of spread and spillover in the society is relatively high,” Lei Zhenglong, the deputy director of the National Health Commission’s Bureau of Disease Control and Prevention, said on Tuesday.

The World Health Organization has listed BA.2 as a variant of concern.

“Based on available data of transmission, severity, reinfection, diagnostics, therapeutics and impacts of vaccines, the group (Technical Advisory Group) reinforced that the BA.2 sublineage should continue to be considered a variant of concern and that it should remain classified as Omicron,” according to the WHO website.

However, some say they believe it’s unlikely BA.2 will trigger a new surge of infection in the U.S. because of the percentage of U.S. citizens who are vaccinated or have natural immunity from prior infections.

“The most likely thing that’s going to happen is that it might extend our tail, meaning it might slow down the decrease in cases. But it’s probably not going to lead to a new wave of cases,” Nathan Grubaugh, an associate professor of epidemiology at the Yale School of Public Health, told NPR.

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