Stealth Omicron, also known as the BA.2 subvariant, is considered sibling to Omicron's BA.1 variant that swept through much of the US. Now, BA.2 is behind a rising number of Omicron cases in Europe and Asia.
BA.2 first overtook BA.1 in Denmark and has become the dominant strain in the Philippines, Nepal, Qatar, India and Denmark. A Danish study found BA.2 was more transmissible but didn't cause more severe disease than BA.1. The latter is corroborated by data from other countries that aren't seeing bumps in hospitalization rates as BA.2 spreads through them.
The study that included 8,500 Danish households also found that BA.2 was better able to infect vaccinated and booster-vaccinated people than BA.1. But once infected with BA.2, the vaccinated and booster-vaccinated were found to spread this subvariant less often to household members compared with the BA.1 subvariant.
What's the theory behind where it came from? BA.2 and BA.1 originated from a common ancestor in South Africa around the same time, but BA.2 took longer to escape the country, speculates Cornelius Romer, a bioinformatician at the University of Basel in Switzerland. Scientists say the BA.2 subvariant may have arisen from viral evolution in an immunocompromised patient with long-term infection or by jumping between animals and humans.
BA.2 is referred to as stealth Omicron because it doesn't have Omicron BA.1's missing spike protein mutations that distinguish the BA.1 variant from previous strains like Delta in PCR testing. Epidemiologist at the Scripps Research Institute, Mark Zeller, thinks BA.2 shouldn't even be considered Omicron and should be labeled its own variant of concern.
This genetic divergence begs the obvious question: will people who were already infected with Omicron BA.1 be immune to BA.2? As seen in Denmark, some places that had high counts of BA.1 infections also had a rise in BA.2 cases. But according to Jesse Bloom, an evolutionary virologist at the Fred Hutchinson Cancer Research Center, antibodies made in response to BA.1 infection are expected to neutralize BA.2 fairly well since the two subvariants have similar binding regions. Hopefully BA.2 won't cause too much of a stall in the Omicron wave petering out.