A C.D.C. airport surveillance program found the earliest known U.S. cases of Omicron subvariants.

Eligible vacationers might volunteer to supply a self-collected nasal swab on the airport. Swabs from 5 to 25 vacationers — from the identical flight or nation — had been added to a single tube after which examined for the virus utilizing P.C.R. testing. Optimistic samples had been then sequenced to find out which model of the virus was current.

Between Sept. 29 and Jan. 23, 10 % of 161,000 eligible vacationers enrolled within the research, and 1,454 pattern swimming pools had been examined for the virus. Regardless of the preflight testing requirement, greater than 15 % of the swimming pools had been constructive.

This comparatively excessive positivity fee could point out that vacationers had been early in the middle of their infections — and thus had viral masses too low for some assessments to detect — after they took their predeparture assessments, or that they contracted the virus within the time between being examined and touchdown in the USA, the researchers say. Individuals may additionally have submitted fraudulent check outcomes.

Earlier than Nov. 28, almost the entire constructive pattern swimming pools contained the Delta variant, the researchers discovered. (The one exception was a constructive pattern whose precise genetic lineage couldn’t be decided.)

After that, nonetheless, Omicron rapidly got here to dominate; from Nov. 28 to Jan. 23, two-thirds of the constructive samples had been Omicron. A lot of the Omicron samples had been the BA.1 subvariant, which was initially probably the most prevalent model worldwide. BA.1 stays the most typical lineage in the USA, although BA.2 has been gaining floor and now accounts for 35 % of infections, in keeping with C.D.C. estimates.

However the researchers discovered the BA.3 subvariant in a pool of samples collected from vacationers arriving from South Africa on Dec. 3. They reported the discovering in GISAID, a world repository of viral genomes, on Dec. 22. It was the primary reported case of BA.3 in the USA; it could be greater than a month earlier than the subsequent was reported.

The airport program additionally detected the primary recognized U.S. case of BA.2, in samples collected from South African vacationers on Dec. 14. The researchers reported the discovering every week earlier than the subsequent U.S. case of BA.2 was reported.

“It is a new instrument within the C.D.C. instrument equipment that works, and we’ve proven it’s efficient and it may be layered with all of our different mitigation measures,” Dr. Friedman stated.