BA.2 Omicron subvariant is now dominant among US Covid-19 infections

The BA.2 subvariant of Omicron is now the dominant Covid-19 variant in the US, estimated to represent more than half of coronavirus variants circulating in the country, according to federal health officials.

The US Centers for Disease Control and Prevention estimates that the so-called “stealth” Omicron variant was discovered in nearly 55 per cent of samples that underwent genetic sequencing within the last week, the agency announced on 28 March.

Scientists have attributed BA.2’s rapid growth to its several key mutations; the gene for the spike protein on its surface has eight mutations not found in BA.1. Researchers have determined that such mutations have made BA.2 more transmissible, not that it necessarily evades antibody protection or causes more-severe illness.

BA.2’s relatively rapid spread in the US – roughly doubling its proportion among circulating variants every two weeks – follows a surge of new infections in China and across some parts of Europe within recent weeks, while states and cities across the US relax public health measures, including mask and vaccine requirements.

About two months ago, the subvariant made up less than 1 per cent of confirmed US infections.

Earlier this month, White House chief medical adviser Anthony Fauci predicted a likely “uptick” of BA.2-related infections, as the relatively low number of confirmed infections mirrored the lull in Europe that preceded a current wave.

He told ABC’s This Week on 20 March that now is “no time at all to declare victory, because this virus has fooled us before and we really must be prepared for the possibility that we might get another variant.”

“We don’t want to be caught flat-footed on that,” he said.

BA.2 makes up nearly three-quarters of new Covid infections in New England, according to the CDC.

Health officials continue to stress that existing vaccine regimens against emerging Covid variants provide strong protections against severe illness, hospitalisation and death.

Roughly 65 per cent of Americans, or more than 217 million people, are considered fully vaccinated, according to the CDC. Nearly 97 million people from that group have received at least one “booster” dose. Roughly 23 per cent of the US has not received any vaccine doses.

Drugmakers Pfizer and Moderna filed applications for emergency use authorisation with the US Food and Drug Administration for a second “booster” dose of their vaccines.

On 29 March, the agency approved fourth doses for people ages 50 and older. Americans with moderately or severely weakened immune systems also are eligible.

Meanwhile, the White House has warned that the federal government has exhausted Covid-19-related funding to be able to buy additional booster doses and fund Covid testing without congressional action.

At the same time, requirements around vital protections – like wearing well-fitting N95 or K95 masks indoors and in crowded spaces – have largely dropped across the US.