The coronavirus mutant widely known as "stealth omicron" is now causing more than a third of new omicron cases around the world, but scientists still don't know how it could affect the future of the pandemic.
Researchers are slowly revealing clues about the strain, a descendant of omicron known as BA.2, while warily watching it become ever more prevalent.
"We're all keeping an eye on BA.2 just because it has done particularly well in some parts of the world," including parts of Asia, Africa and Europe, said Dr. Wesley Long, a pathologist at Houston Methodist in Texas.
This week, a technical advisory group for the World Health Organization advised public health authorities to monitor it as a distinct omicron strain.
Early research suggests it spreads faster than the original omicron and in rare cases can sicken people even if they've already had an omicron infection. There's mixed research on whether it causes more severe disease, but vaccines appear just as effective against it.
Overall cases are falling in some places where the variant is becoming more prevalent, offering some hope that the latest troubling version of the virus won't send cases skyrocketing again as experts try to learn more.
BA.2 has been found in more than 80 countries and all 50 U.S. states.
In a recent report, the WHO said BA.2 was dominant in 18 countries and it represented about 36% of sequenced omicron cases submitted in the most recent week to a publicly available international database where scientists share coronavirus data. That's up from 19% two weeks earlier.
In the United States, BA.2 caused about 4% of COVID cases during the week ending Feb. 19, according to the Centers for Disease Control and Prevention. The percentage was lower in some regions and higher in others – hitting about 7% in New England.
BA.2 has lots of mutations. It's been dubbed "stealth" because it lacks a genetic quirk of the original omicron that allowed health officials to rapidly differentiate it from delta using a certain PCR test. So while the test can detect a BA.2 infection, it looks like a delta infection.
Initial research suggests BA.2 is more transmissible than the original omicron — about 30% more contagious by one estimate.
But vaccines can protect people from getting sick. Scientists in the United Kingdom found that they provide the same level of protection from both types of omicron.
A bout with the original omicron also seems to provide "strong protection" against reinfection with BA.2, according to early studies cited by the WHO.
But getting BA.2 after infection from the original omicron strain is possible, says new research out of Denmark. Study authors noted 187 total reinfections, including 47 with BA.2 occurring shortly after a bout the original strain, mostly in young, unvaccinated people with mild disease. They concluded that such reinfections do occur but are rare. Like other early studies on BA.2, this one has been posted online but not reviewed by independent scientists.
Does BA.2 make you sicker?
A Japanese lab study suggests that it could, based on experiments with hamsters. Researchers concluded that the risk for global health "is potentially higher" from BA.2 and proposed that it be given its own Greek letter – a designation for globally significant "variants of concern." WHO's technical group said BA.2 should remain under the omicron umbrella.
Though the severity experiments were conducted in animals, the study is "not something to discount," said Dr. Eric Topol of Scripps Research Translational Institute. "We should keep an open mind and keep assessing this."
But scientists are finding something different when they look at people. An initial analysis in Denmark showed no differences in hospitalizations for BA.2 compared with the original omicron, which tends to generally cause milder disease than the delta variant. More recently, researchers in South Africa found much the same: a similar risk of hospitalization and severe disease with the original omicron variant and BA.2.
"We always have to interpret studies in animals with caution," Long said. "I place more weight in studies of actual patients and what they're experiencing."
How will BA.2 affect the pandemic?
No one knows for sure.
COVID-19 cases are dropping globally, including in some of the places where BA.2 is prevalent.
"The timing of the upswings and downswings in cases remains unclear," said Louis Mansky, director of the Institute for Molecular Virology at the University of Minnesota.
It's difficult for researchers to predict how much BA.2 will change caseloads because it is spreading in communities with varying levels of protection from vaccines and prior infections. Some experts believe BA.2 is unlikely to spark new surges but may slow COVID declines in some places.
WHO officials stress that the pandemic isn't over and urge countries to remain vigilant.
Doctors said individuals should do the same and remember that vaccines and boosters offer excellent protection against the worst effects of COVID-19, no matter the variant.
"For people who aren't boosted, please get boosted. For people who aren't vaccinated, it's never too late," Long said. "Your best defense against COVID is still the vaccine."