Reinfection with the BA.2 subtype of the Omicron variant of coronavirus can occur shortly after initial infection with the BA.1 strain, according to a Danish study.

With the surge of Omicron subvariants BA.1 and BA.2, a large number of reinfections from earlier cases have been observed, the researchers said. This has raised questions of whether BA.2 specifically can escape the natural immunity acquired shortly after a BA.1 infection, they said. The researchers from Statens Serum Institut (SSI) in Denmark selected a subset of samples from over 1.8 million cases of infections in the period from November 22, 2021, until February 11, 2022.

The yet-to-be peer-reviewed study, posted on the preprint repository MedRxiv, identified a total of 187 reinfection cases. The researchers found 67 cases in which the same individual had become infected twice at a 20-60-day interval, and where both infections were due to Omicron subtypes. In 47 of the cases, the affected individual first became infected by BA.1 and then by BA.2, they said.

The majority of the infected were young and unvaccinated, and most experienced mild symptoms during their infections, according to the researchers. The team noted that the difference between the severity during their first and second infection was negligible. None of the infected individuals had become seriously ill, and none required admission to hospital, they said.

"The study shows that infection with two different Omicron subtypes is possible," the authors of the study noted. "This seems to occur relatively rarely in Denmark, and reinfections have mainly affected younger unvaccinated individuals, they added.