Though the coronavirus mutant BA.2, widely known as ‘stealth Omicron,’ is now causing more than a third of new Omicron cases around the world, two new studies are helping to show how well human immunity is defending against this strain. BA.2 has been found in more than 80 countries and all 50 US states. In a recent report, the World Health Organisation (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.
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 UK 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.
“As of now, I don’t think that we need to sound a global alarm. But I do think that we need to pay attention to BA.2 because it does appear to have a growth advantage over BA.1 (the original Omicron variant),” says Dr Dan Barouch, director of the Center for Virology and Vaccine Research at Beth Israel Deaconess Medical Center in Boston. Like the BA.1 strain, BA.2 also has features that help it escape some immunity from vaccines and from most monoclonal antibody treatments, though recent boosters improve personal protection and antiviral pills are still expected to work against this subvariant.
Now new studies are providing some reassurance that while BA.2 may overtake its genetically distant cousin, it won’t likely lead to greater numbers of hospitalisations and deaths. “The situation that we’re seeing on the ground, and I get this from talking to a number of my colleagues who actually do the genomic surveillance, is BA.2 is kind of creeping up in terms of numbers, but it’s not the meteoric rise that we saw with BA.1,” said Angela Rasmussen, a virologist at the Vaccine and Infectious Disease Organisation at the University of Saskatchewan in Canada.
That’s because in many countries like the US, the UK, and Denmark, BA.2 has hit speed bumps left in its wake by BA.1, which was already very contagious. “It’s so soon after that initial BA.1 peak that you have a lot of people who were either vaccinated or boosted ... [or] got Omicron, and so right now all of those people will have relatively high titers of antibodies, neutralising antibodies that will protect against infection,” Rasmussen says.
The new studies are preprints, which means they were posted to an online library of medical research before being reviewed by outside experts and published in medical journals. 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. The relief is that some experts believe BA.2 is unlikely to spark new surges but may slow Covid declines in some places.