‘Stealth Omicron’ is now in the U.S.—but experts say so far it doesn’t seem more dangerous

January 25, 2022, 11:40 AM UTC

The coronavirus variant known as “stealth Omicron” has, inevitably, made its way to the U.S. However, there is still no reason to fear it will prove worse than the kind of Omicron already taking hold across the country.

Cases of the BA.2 sub-strain have been identified in Washington state, Texas and elsewhere. So far, according to Washington authorities, there are fewer than 100 confirmed cases in the country.

That’s a far cry from the situation in Denmark, where BA.2 now accounts for around 65% of new infections—a significant surge, given that Denmark is experiencing record case numbers of around 40,000 a day. In the U.K., health authorities said Friday that BA.2 was a “variant under investigation” that is accounting for a growing proportion of cases, with 426 now confirmed.

BA.2 is known as “stealth Omicron” because it lacks a particular mutation that makes BA.1—the type of Omicron that’s been spreading like wildfire over the last couple months—relatively easy to identify using PCR tests. (Contrary to some reports, the lack of this mutation does not mean BA.2 evades tests; it just makes it harder to correctly classify.)

Indeed, BA.2 seems to have dozens of mutations different to those found in the original Omicron—a fact that had led some to recommend its classification as a different variant. However, it remains unclear how those different mutations might affect BA.2’s virulence or transmissibility.

Danish authorities, who are best placed to analyze its effects, say there is so far no different in hospitalization rates between BA.1 and BA.2, and expect vaccines will probably block serious illness caused by BA.2.

Francois Balloux, director of the Genetics Institute at University College London, said Monday that sub-strains appear all the time—Delta had over 200 of them before Omicron displaced it. Importantly, he noted that few of the mutational differences between the Omicron sub-strains should affect the ability of antibodies to recognize the virus.

“As such, it is anticipated that infection by either sub-lineage should provide robust immunity against the other one, as well as against itself,” he said. “There is no evidence so far that BA.1 and BA.2 are different in respect of immune escape, virulence or the age profile they preferentially infect. At this stage, BA.1 and BA.2 can be considered as two epidemiologically largely equivalent sub-lineages of Omicron.”

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