Geneva, Switzerland—How worrisome is the emerging BA.2 form of the Omicron coronavirus variant that has gained dominance in Denmark and several other countries?

While it does not seem to be any more severe than the original BA.1 form, it might be even more transmissible than the original Omicron variant.

Boris Pavlin, MD, of the World Health Organization (WHO) COVID-19 Response Team, said at an online briefing at the beginning of February that BA.2 does not appear to cause more severe disease and that vaccines provide equivalent protection against the different forms of Omicron.

Dr. Pavlin pointed out that, looking at data from Denmark, he expects BA.2 to replace BA.1 globally. "Looking at other countries where BA.2 is now overtaking, we're not seeing any higher bumps in hospitalization than expected," he stated. In addition to Denmark, the subvariant is already becoming dominant in the Philippines, Nepal, Qatar, and India, he advised.

"Vaccination is profoundly protective against severe disease, including for Omicron. BA.2 is rapidly replacing BA.1," Dr. Pavlin said. "Its impact is unlikely to be substantial, although more data are needed."

At the same time, a preprint of a Danish study suggests that BA.2 is more transmissible than the more common BA.1, including when it comes to infecting vaccine recipients.

"The Omicron SARS-CoV-2 variant of concern (VOC lineage B.1.1.529), which became dominant in many countries during early 2022, includes several subvariants with strikingly different genetic characteristics," Danish authors wrote in the study, which had not been peer-reviewed at the time. "Several countries, including Denmark, have observed the two Omicron subvariants: BA.1 and BA.2. In Denmark the latter has rapidly replaced the former as the dominant subvariant."

Researchers used nationwide Danish data to estimate the transmission dynamics of BA.1 and BA.2 within Danish households in late December 2021 and early January 2022.

The authors reported that—among 8,541 primary household cases, of which 2,122 were BA.2—they identified 5,702 secondary infections among 17,945 potential secondary cases during a 1-to-7-day follow-up period. They calculated that the secondary attack rate was estimated as 29% in households infected with Omicron BA.1 compared with 39% infected with BA.2.

"We found BA.2 to be associated with an increased susceptibility of infection for unvaccinated individuals (Odds Ratio (OR) 2.19; 95%-CI 1.58-3.04), fully vaccinated individuals (OR 2.45; 95%-CI 1.77-3.40) and booster-vaccinated individuals (OR 2.99; 95%-CI 2.11-4.24), compared to BA.1," the study notes. "We also found an increased transmissibility from unvaccinated primary cases in BA.2 households when compared to BA.1 households, with an OR of 2.62 (95%-CI 1.96-3.52)."

They added, however, that the pattern of increased transmissibility in BA.2 households "was not observed for fully vaccinated and booster-vaccinated primary cases, where the OR of transmission was below 1 for BA.2 compared to BA.1."

Researchers concluded that Omicron BA.2 is inherently substantially more transmissible than BA.1 and further explain that it "possesses immune-evasive properties that further reduce the protective effect of vaccination against infection, but do not increase its transmissibility from vaccinated individuals with breakthrough infections."

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