On the heels of concerning new lab and animal experiments suggesting that BA.2 may be capable of causing more severe disease than the original Omicron strain, two new studies are helping to show how well human immunity is defending against this strain in the real world.
BA.2 is about 30% more transmissible than the original Omicron variant, BA.1, according to early studies from the UK and Denmark, and it is now causing about 1 in 5 Covid-19 cases worldwide, according to the World Health Organization.
Even as Covid-19 cases have been dropping around the world, the relative proportion of cases caused by BA.2 has been increasing. It is outcompeting the original Omicron strain in at least 43 countries, prompting fears of another devastating pandemic wave.
“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,” 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 hospitalizations 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 Organization at the University of Saskatchewan in Canada.
That’s because in many countries like the US, 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, neutralizing 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.
No increase in hospitalizations
The first new study comes from South Africa, where BA.2 grew rapidly, rising from 27% to 86% of new Covid-19 infections over the course of a single week in February. Researchers looked at cases tied to more than 95,000 positive Covid-19 tests. Among these, roughly equal proportions of people were hospitalized for their infections – roughly 3.6% of people who had presumed BA.2 infections compared to 3.4% of those with signs infections caused by BA.1.
After researchers accounted for things that might influence a person’s risk of severe disease, such as older age, they found no difference in the risk for hospitalization between people infected by BA.1 and those infected by BA.2. Roughly one-quarter of people hospitalized with both BA.1 and BA.2 infections were fully vaccinated.
Those findings echo hospitalization studies from Denmark, where BA.2 is also the predominant cause of Covid-19 infections.
The lead researcher on the South African study, Dr. Nicole Wolter, says that while it’s difficult to say how the South African experience with this variant may translate to other countries, what they’re seeing from BA.2 after their fourth wave isn’t a second peak, but a longer tail.
“We have seen an extended wave which has currently plateaued at a higher level than we have seen in previous inter-wave periods,” Wolter wrote in an email to CNN. “This may however also be influenced by schools opening following the December holiday period and a general relaxing of restrictions,” wrote Wolter, who is a principal medical scientist at the National Institute for Communicable Diseases in Johannesburg.
Though cases have stayed at a high level, and most are now caused by BA.2, hospitalizations have continued to decline.
What about reinfection risk?
The second new study, from Denmark’s Statens Serum Institut – the country’s equivalent to the US Centers for Disease Control and Prevention – looked at the risk of reinfection with BA.2 after recovery from a Covid-19 infection caused by other recent variants, including Delta and BA.1.
The study found that people who’ve recently had a Covid-19 infection caused by Omicron or Delta can be reinfected by the emerging BA.2 subvariant, but such cases appear to be uncommon, afflict mostly those who are unvaccinated and result in mostly mild infections.
BA.2 is currently the dominant cause of Covid-19 in Denmark. It overtook BA.1 during the second week of January there.
To look at reinfection risk, researchers combed through more than 140,000 viral genomes that were sequenced from infections during the period when Omicron became dominant (late November to mid-February 2022) to find people who had a new positive test 20 to 60 days after a previous one.
They found a total of 263 reinfections, with 190 of those caused by the BA.2 variant. In 140 cases, the person was reinfected by BA.2 after an infection caused by the Delta variant. There were 47 cases where people were first infected by BA.1 (the original Omicron variant) followed by the BA.2 subvariant.
Researchers then did a more in-depth analysis of those 47 reinfections where BA.2 followed BA.1. Most of the individuals who were reinfected were young – 30 were under age 20. None of the reinfected individuals was over age 40 and nearly all – 42 of the 47 individuals – were unvaccinated.
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For the most part, reinfections were mild; 28 people had no symptoms or mild symptoms. Five people experienced symptoms that were characterized as moderate, akin to flu-like symptoms. There were no hospitalizations or deaths reported among reinfected individuals.
The study shows that “reinfection can happen with people who recently recovered from BA;1, but it’s pretty rare. And in all those cases, it wasn’t severe,” says Rasmussen, who reviewed the study but was not involved in the research.
What that means she says is that while BA.2 reinfection is a risk, it is a small risk relative to the entire population, and “that recent immune boost caused by either boosters, or recent Omicron infection is largely going to protect most of the population against it,” she said.