Omicron subvariant's spread stalls in US: 6 updates

Early research from the U.K. offers new insights into the omicron subvariant's spread relative to the original strain.

As of Feb. 7, England had confirmed 7,194 cases of BA.2, according to a Feb. 11 report from the U.K. Health Security Agency. A preliminary analysis of contract-tracing data for these cases suggests the subvariant's mean serial interval — or the average time from symptom onset of an initial case to symptom onset of their identified contacts — is about half a day shorter than BA.1. This figure sat at 3.27 days for BA.2, compared to 3.72 days for BA.1. For context, the mean serial interval for delta was 4.09 days.

"The serial interval suggests the time between primary and secondary infections is shorter, which could contribute to the increased growth rate of BA.2," researchers said.

Five more updates:

1. The World Health Organization said it expects cases of the omicron subvariant to increase globally due to its growth advantage over BA.1. 

"BA.2 is more transmissible than BA.1, so we expect to see BA.2 increasing in detection around the world," Maria Van Kerkhove, PhD, the WHO's technical lead for Covid-19, said during a Feb. 8 media briefing. "There isn't any indication to suggest that there's a difference in severity of BA.2 compared to BA.1, but it's still very early days."

2. As of Feb. 8, 48,622 BA.2 sequences have been identified in 67 countries, the majority of which are in Denmark and the U.K.

3. As of Feb. 1, 127 BA.2 cases had been confirmed in the U.S., with nearly half of all states reporting the subvariant's presence, according to CNBC. However, the weekly number of sequences identified in the U.S. has fallen since mid-January, according to the U.K. Health Security Agency's report. The U.S. confirmed 191 BA.2 sequences in the week of Jan. 17, which fell to 116 in the week of Jan. 24. In the week of Jan. 31, just four sequences were confirmed, according to supplemental data from the report.

"Notably, its spread has stalled in the U.S. and other countries," Eric Topol, MD, director of the Scripps Research Translational Institute in La Jolla, Calif., tweeted Feb. 11 in response to the report's findings. 

4. It's still unclear whether BA.2 can cause reinfections among people who were infected by BA.1. U.K. researchers said they have not detected sequencing to confirm such a reinfection at any interval to date in England, though only a limited proportion of virus samples are sequenced. 

5. COVID-19 vaccines appear similarly effective at preventing symptomatic infection from BA.1 and BA.2, according to the U.K. Health Security Agency.

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