People infected with the highly contagious Omicron variant of COVID are less likely to contract BA.2 subvariant, according to a study.

Omicron, which has rapidly spread worldwide, carries numerous mutations in key regions and is associated with increased transmissibility and immune escape. The variant has recently been divided into three subvariants with substantial genomic differences, in particular between Omicron BA.1 and BA.2. While Omicron is on the decline, many countries in Europe and Asia have seen an increase in BA.2 cases.

With the surge in BA.2, a large number of reinfections from earlier cases has been observed, raising the question of whether the subvariant specifically can escape the natural immunity acquired shortly after a BA.1 infection. "We provide evidence that Omicron BA.2 reinfections do occur shortly after BA.1 infections but are rare," said corresponding author Morten Rasmussen, from the Department of Virus and Microbiological Special Diagnostics, at Statens Serum Institut, Denmark.

While the reinfection rate appears to be low given the high number of positive SARS-CoV-2 tests during the study period, it "still highlights the need for continuous assessment of the length of vaccine-induced and/or natural immunity", Rasmussen said. To explore, a team of several Danish and Australian researchers selected a subset of samples from more than 1,8 million cases of infections in the period from November 22, 2021, until February 11, 2022. They selected individuals with two positive samples, more than 20 and less than 60 days apart.

From a total of 187 reinfection cases, they identified 47 instances of BA.2 reinfections shortly after a BA.1 infection, mostly in young unvaccinated individuals with mild disease not resulting in hospitalisation or death. Further, the study showed that Omicron BA.2 reinfections after either Delta or BA.1 initial infections, were mainly observed among young individuals below the age of 30 and the majority of these cases were not vaccinated. This emphasises the enhanced immunity obtained by the combination of vaccination and infection compared to infection-induced immunity only, Rasmussen said.