In this July 19, 2021, file photo, a passenger, right, gets off the Amtrak Vermonter passenger train after it arrived at the Montpelier station in Berlin, Vermont. Credit: Wilson Ring / AP

BA.2, the newest subvariant of omicron, is highly transmissible and accounts for 55 percent of COVID-19 infections in the U.S., according to the Centers for Disease Control and Prevention.

“It has been classified as a variant of concern, and it’s causing the majority of cases of COVID worldwide and here in the United States. The studies that have been done on this variant have shown that it has what we call a ‘growth advantage,'” said Dr. Matthew Binnicker, director of the Clinical Virology Laboratory at Mayo Clinic. “In other words, it has a higher transmission rate over prior variants of SARS-CoV-2, the virus that causes COVID-19.”

While the BA.2 variant is extremely contagious, don’t call it a “stealth variant,” Binnicker said.

“It’s not an accurate term. Stealth variant really implies that it’s going undetected and that we’re missing it. Because with prior versions of omicron — the original version of omicron — some labs were able to get an idea of whether it was omicron by a certain type of PCR test, where a component of that test would be negative, but the others would be positive,” Binnicker said. “That was what we called an ‘S gene drop out’ or ‘S gene target failure.’ And BA.2 doesn’t show that kind of result profile. But the PCR tests and the antigen tests are still able to pick up BA.2 similar to BA.1 and all the other prior COVID variants. Stealth variant really is a misnomer, and we are able to detect this virus.”

There may be several contributing factors that have caused the recent surges in places such as Germany and Denmark, as well as the rising number of infections in parts of the U.S., Binnicker said.

“It’s somewhat difficult to tease apart the causes for this, it’s partially due to the virus itself. Also, many of these locations, including in the U.S., have eased many of their COVID restrictions. So I’m sure that’s contributing, as well,” he said.

The BA.2 subvariant is a sublineage of the omicron variant that was first identified in November 2021. There are some unique mutations in BA.2 that differ from the original omicron variant. Binnicker said researchers believe the differences between the two are leading BA.2 to be spread from one person to another more efficiently than prior variants.

“There’s also some specific mutations in this BA.2 variant that might allow it to evade existing immunity more efficiently. And that again, could be from vaccine-induced immunity or from natural immunity, and so that immune evasion may be even more efficient than BA.1,” he said.

While the virus spreads faster, it is not yet known if BA.2 has higher potential for severe disease than other variants.

Story by Deb Balzer, Mayo Clinic News Network.