BA.2 has now been detected in more than 30 states, makes up around 3.9 per cent of new infections, and appears to be doubling quickly, according to the Centers for Disease Control and Prevention’s data tracker.
“If it doubles again to 8 per cent, that means we’re into the exponential growth phase and we may be staring at another wave of Covid-19 coming in the US,” Samuel Scarpino, the manager director of pathogen surveillance at the Rockefeller Foundation, told NPR.
“And that’s of course the one we’re really worried about. We’re all on the edge of our seats,” Mr Scarpino said.
BA.2 is believed to be far more contagious than the earlier Omicron strain, and was blamed for a fresh surge in Denmark.
Yet fears of another Omicron wave in the United States may be averted given vaccination and immunity rates from previous infections.
Nathan Grubaugh, an associate professor of epidemiology at the Yale School of Public Health, told NPR it would more likely lead to a long tail rather than a fresh surge.
“A lot of us were assuming that it was going to quickly take off in the United States just like it was doing in Europe and become the new dominant variant,” Mr Grubaugh said.
Other scientists warn that removing mask mandates will allow the new strain to spread.
The fresh strain also appears to be better at avoiding the immune system’s defences than the original Omicron variant was.
BA.2 is considered a “stealthier” version of Omicron because particular genetic traits make it somewhat harder to detect.
Danish scientists reported this week that preliminary information suggests it may be 1.5 times more contagious than the original variant.
The US is still recording around 100,000 new cases and 2,000 deaths per day from the Omicron surge, according to the CDC’s Covid tracker.