Fast-Spreading Omicron BA.2 Variant Makes Up Nearly 75 Percent of U.S. Cases

The Omicron COVID sub-type BA.2 has been detected in 72.2 percent of COVID samples sequenced in the U.S., according to the most recent estimates from the Centers for Disease Control and Prevention (CDC).

That marks a significant increase from last week's data, when BA.2 was estimated to account for 57.3 percent of samples—revised upwards from 54.9 percent.

BA.2 has been increasing as a proportion of U.S. infections for weeks, though the actual number of U.S. cases remains relatively low. On April 3, the seven-day moving average of new U.S. cases was just over 25,000, down from the mid-January peak of over 800,000.

Elsewhere, though, BA.2 has driven waves of infections. New cases hit record levels in the U.K. recently, with around 4.9 million people estimated to have had COVID in the week ending March 26, according to the country's Office for National Statistics (ONS).

Surging Cases in the U.K.

BA.2 has been studied for months, after scientists noted an increase in the proportion of cases it accounted for since the start of this year. Based on existing data, BA.2 is thought to have an increased growth rate of about 75 percent, compared to the previously dominant BA.1 variant, according to a technical briefing by the U.K.'s Health Security Agency (HSA) on March 25.

It was stated at the same briefing that there was no evidence of a greater risk of hospitalization following infection with BA.2 compared to BA.1.

A vaccine surveillance report by the U.K. HSA on March 31 noted that vaccine effectiveness against both symptomatic disease and hospitalization was similar for BA.1 and BA.2.

One key characteristic of BA.2 is its ability to reinfect people who have already been infected with BA.1. Data shows that a number of people have been infected with BA.2 after BA.1, meaning it's possible to catch Omicron twice in a matter of months.

It remains to be seen whether BA.2 will drive a surge in cases in the U.S. in the same way it has in the U.K., but experts have said that is possible.

Asked whether the U.S. would see another such rise in cases, Dr. Ayman El-Mohandes, dean of the City University of New York Graduate School of Public Health and Health Policy, told NBC New York: "To be honest with you, unless we take more precautions and are more responsible, it's very likely that we will.

"There are already 17 counties around the country that have reached the level where the mask mandate by the CDC is enacted and of those, three are in New York."

COVID test site
A COVID test site seen in Manhattan, New York City, on March 31, 2022. Prevalence of the Omicron BA.2 sub-type in the U.S. has increased in recent weeks. Spencer Platt/Getty