Stealth BA.2 Omicron Variant Can Reinfect People Who Had BA.1, Study Finds

The COVID Omicron sub-variant BA.2, informally dubbed the "Stealth Omicron" variant, is able to reinfect people who have already had Omicron, a new study suggests.

BA.2 has been under close observation by scientists for several weeks after it was reported to be responsible for an increasing proportion of cases in countries such as Denmark, the U.K., South Africa, and, to a lesser extent, the U.S.

Data suggests that BA.2 is even more transmissible than the already highly contagious BA.1 Omicron variant that has been responsible for the majority of Omicron cases worldwide.

One of the key unknown characteristics of BA.2 is whether it is different enough from BA.1 to be able to cause illness in people who have already had the latter—raising the possibility that people could catch Omicron twice in the few months the COVID variant has existed.

Research from the U.K. earlier this month had found no evidence of any such cases. But a new study from Denmark's government-affiliated research lab the Statens Serum Institut (SSI) suggests it is indeed possible and has happened dozens of times in Denmark.

It should be noted that the study is a pre-print and has not yet gone through the validation process of peer review.

The researchers studied data from over 1.8 million Danish COVID cases from November 22, 2021, until February 11, 2022, and selected people who had tested positive for COVID twice in that time, based on two positive samples returned more than 20 and less than 60 days apart.

This resulted in a total of 1,739 possible reinfection cases. Upon further analysis using genetic sequencing, the researchers found that there were 47 instances of people becoming infected with the BA.2 Omicron variant shortly after they had caught BA.1.

The cases were "mostly in young unvaccinated individuals with mild disease not resulting in hospitalization or death," the authors noted. The median age was 15 years.

Of those 47, the vast majority, 42, were unvaccinated. None of the people who had caught BA.2 after BA.1 had been hospitalized or died during the follow-up study period. Most reported mainly mild symptoms.

The study authors concluded that the BA.1 to BA.2 reinfection rate "appears to be low" but still highlights the need for continued assessment of how long COVID immunity lasts.

They added that since the majority of people reinfected were not vaccinated, the study further emphasizes "the enhanced immunity obtained by the combination of vaccination and infection compared to infection-induced immunity only."

Ian Jones, professor of virology at the University of Reading in the U.K., told Newsweek he was not particularly surprised by the results. "I don't think it changes much," he said. "Just as Omicron can infect after Delta, so BA.2 can infect after BA.1." He said the "important point" was that reported symptoms were mild.

Concern about BA.2 was raised last week after data from Japan suggested the variant made animals sicker than BA.1 in lab tests, though experts noted that data from animal tests is not always useful for making predictions about humans.

Holding COVID test
A stock photo shows someone holding a positive COVID lateral flow test. Research suggests it is possible to catch the Omicron sub-variant BA.2 after getting the Omicron sub-variant BA.1. Ladanifer/Getty