'Stealth Omicron' Has Arrived in the US

Cases of an omicron subvariant have popped up in at least 40 countries, including the US. Health officials in California's Santa Clara County confirmed two cases of "stealth omicron," or BA.2, on Tuesday, adding to 11 existing cases in the state as of Monday, reports the San Jose Mercury News. Cases have also been reported in Texas, New Mexico, and Washington state, per USA Today. Here's what we know so far:

• Limited data: Some 8,040 BA.2 sequences have been reported in 40 countries since Nov. 17, with 6,411 of those in Denmark, 530 in India, and 426 in the UK, the UK's Health Security Agency said Friday. However, the AP reports nearly 15,000 genetic sequences of BA.2 have been uploaded to a global data-sharing platform, with 96 of those coming from the US.

• New mutations: For now, BA.2 is considered a subset of omicron, BA.1, as with BA.1.1 and BA.3, but it could get its own Greek letter name if found to be a significant "variant of concern." (The UK has labeled it a "variant under investigation.") With its new mutations, the differences between BA.1 and BA.2 are actually greater than those between the original SARS-CoV-2 strain and the alpha variant, the CBC reports.
• The unknowns: It's unclear where BA.2 first originated. It may only appear most prevalent in Denmark because of a "robust program of sequencing the virus's genome," per the Washington Post. It's also unknown whether BA.2 will escape the protection offered by prior infections, including with BA.1, or existing vaccines. But "I don't think there's any reason to think this one is a whole lot worse than the current version of omicron," says Dr. Robert Garry of Tulane University.

• Perhaps more contagious: BA.2 is overcoming BA.1 in some countries. It now accounts for 65% of cases in Denmark, per the Post. It's also now more prevalent than BA.1 in South Africa, per USA Today. "Early analyses suggest an increased growth rate compared to BA.1; however, growth rates have a low level of certainty early in the emergence of a variant," notes the UK's Health Security Agency.
• Why stealthy? Most global sequences of the omicron variant include a deletion in the S gene, which triggers what's known as "S gene target failure" in PCR tests. This is how experts know they're dealing with omicron. But as with delta, the so-called stealthy BA.2 doesn't have this deletion, meaning S gene target failure can't be used to identify it (though it will still be detected by PCR tests). Officials instead have to rely on full genetic sequencing.

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• Taking priority: Given rising cases in many countries, "investigations into the characteristics of BA.2, including immune escape properties and virulence, should be prioritized independently (and comparatively) to BA.1," according to the World Health Organization. Meanwhile, the California Department of Public Health is continuing to urge people to get vaccinated as "new variants will continue to evolve as long as there are large pockets of unvaccinated people," per the Mercury News.

(More omicron variant stories.)