Good news for the pandemic-weary people as a study shows that previous infection with the widely circulating BA.1 version of the Omicron Covid variant provides strong protection against its relative BA.2, which is spreading now.
The study, which was available on the preprint server medRxiv, has not yet been peer reviewed.
The results of the study suggest that BA.2 is unlikely to cause a major wave of infections in communities that have experienced a BA.1 wave. "When I read it, I said, 'This is definitely reassuring,'" says Eric Topol, a genomicist at Scripps Research in La Jolla, California.
Ever since the Omicron variant was identified in November 2021, mostly its BA.1 subvariant has dominated, while the number of cases attributed to its sister subvariant, BA.2, has begun to rise in the past few weeks.
The two strains diverged around a year ago — months before scientists identified them — and they have since accumulated substantial genetic differences, leading scientists to wonder whether BA.1 infection would provide protection against BA.2.
Earlier this month, researchers found that the BA.2 subvariant spreads more quickly than BA.1. It also causes more severe disease in hamsters — a common model for studying respiratory illnesses — than does the BA.1 subvariant 2, raising concerns that it would cause another spike in cases.
Troels Lillebaek, a molecular epidemiologist at the State Serum Institute in Copenhagen, and his colleagues studied Denmark's medical registries, in which nearly two million Danish residents tested positive for COVID-19 from late November to mid-February. But only 1,739 people had results classed as a reinfection: two positive tests separated by 20–60 days.
The researchers sequenced viral samples from 263 of these people and found that only 47 had contracted BA.2 after an infection with BA.1. By contrast, 140 people had contracted BA.2 after infection with the Delta variant.
Proliferating BA.2 variant
The BA.2 subvariant has been proliferating in Denmark since the start of this year, and currently comprises about 88% of all coronavirus cases. But Lillebaek says the wave of BA.1 that preceded BA.2 is offering protection. "There's a build-up of immunity at the moment that is preventing a disaster," he says.
Sarah Otto, an evolutionary biologist at the University of British Columbia in Vancouver, says these findings are consistent with other recent studies. Earlier, two laboratory studies have shown that antibodies against BA.1 can protect cells from infection with BA.2. even a survey in the United Kingdom on reinfections in early February did not identify any cases in which BA.2 infection followed BA.1 infection.
"If BA.2 arrives in a community late, when the BA.1 Omicron wave is nearly over, immunity by Omicron infection and/or by boosting is likely to keep BA.2 from driving a second Omicron wave," Otto said.
Hope for a reprieve
Topol says the study's results mean that many communities can relax. "Instead of thinking that [BA.2] is the new bad variant, I think we can put that aside. I see it as not a worry," he said.
Lillebaek says the study supports the idea that vaccines provide protection against Omicron, including BA.2. "It's predominantly young, unvaccinated persons where we see this reinfection with BA.2. It indicates that vaccination does give you some protection," he noted.
