WASHINGTON: Two new Omicron subvariants are on the rise in the United States, adding to concern of health experts whether they may fuel a summer surge in Covid-19 cases.
The subvariants, known as BA.4 and BA.5, were estimated to make up nearly 13 per cent of all new US Covid-19 cases in the latest week ending June 4, the US Centres for Disease Control and Prevention (CDC) said on Tuesday.
BA.4 made up 5.4 per cent of the new cases, while BA.5 made up 7.6 per cent, according to CDC estimates.
The two subvariants represented the highest percentage of cases in a region that includes Texas, New Mexico, Oklahoma, Arkansas and Louisiana, according to the CDC, Xinhua news agency reported.
Another subvariant BA.2.12.1, which remains the dominant variant in the country, made up 62.2 per cent of all new US cases in the past week following months of steady growth.
The BA.4 and BA.5 subvariants were added to the World Health Organization's monitoring list in March and designated as variants of concern by the European Centre for Disease Prevention and Control.
CDC data showed they were present in all US regions.
Evidence suggests the new subvariants are yet-more contagious versions of Omicron, which may be able to dodge antibodies from earlier infection well enough to trigger a new wave, health experts said.
"BA.4 and 5 may end up becoming the dominant Omicron lineages in the coming weeks or months," said Nathan Grubaugh, an associate professor of epidemiology at Yale University's School of Public Health.
The United States is currently averaging about 108,000 Covid-19 cases and 300 deaths each day, the latest CDC data showed.
Health experts believe the real number is significantly higher as increasing at-home testing results had not been tracked.
Some epidemiologists warned surging cases may put people with compromised health at risk and can also expose many infected people to long-term health problems.