Why calling the highly contagious COVID-19 omicron BA.2 a 'stealth variant' is a misnomer

BA.2, the newest subvariant of omicron, is highly transmissible and accounts for 55% 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,'" says 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," says Dr. Binnicker.

"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," says Dr. Binnicker. "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., says Dr. Binnicker.

"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 says.

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. Dr. Binnicker says 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 says.

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