The BA.4 and BA.5 sub-variants of Omicron are estimated to make up about 8.3 per cent and 13.3 per cent of the coronavirus variants in the United States as of Jun 11, the US Centers for Disease Control and Prevention (CDC) said on Tuesday (Jun 14).
The two fast-spreading sublineages were added to the World Health Organization's monitoring list in March and have also been designated as variants of concern in Europe.
The European Union's disease prevention agency said on Monday the new subvariants are spreading more quickly than other variants, which could lead to more hospitalisations and deaths as they become dominant in the continent.
"I'm very concerned," said Dr Gregory Poland, head of Mayo Clinic's vaccine research group in Rochester, Minnesota, adding that data from South Africa shows the subvariants are better at circumventing immunity provided through vaccines and previous infections.
Vaccines or previous infection may provide a benefit against death or severe illness, but BA.4 and BA.5 could potentially cause a surge in infections during the summer, just as kids go back to school and people's second booster may have worn off, he said.
BA.4 made up 5.4 per cent of the variants in the United States for the week of Jun 5, according to CDC estimates, while BA.5 made up 7.7 per cent of the variants.
The seven-day moving average of US COVID-19 cases stood at 105,615 as of Jun 11, up 6.7 per cent from a week earlier.