A new, highly transmissible subvariant of Omicron is spreading rapidly across the US, as Covid-19 cases in the country are ticking up again.
The new strain, called BA.2.12.1, made up 36.5 per cent of new Covid-19 cases in the country in the week ending April 30, according to data released Tuesday by the US Centers for Disease Control and Prevention (CDC), Xinhua news agency reported.
The data increased from 26.6 per cent a week before, and 16.7 per cent two weeks prior, CDC data showed.
The majority of Covid-19 cases in the US - around 61 per cent - are still caused by BA.2., which has become the country's dominant variant since March.
"BA.2.12.1 has been noted to be of higher concern, given additional mutations," said the department.
BA.2.12.1 appears to be about 25 per cent more transmissible over the BA.2 subvariant, said CDC Director Rochelle Walensky, adding that additional evaluation is currently underway to understand the impact of BA.2.12.1 on vaccine effectiveness.
Besides BA.2.12.1, a pair of new Omicron subvariants has emerged, raising the possibility that people infected by earlier Omicron strains can get reinfected.
The pair, BA.4 and BA.5, have gained increasing attention in South Africa as weekly Covid-19 cases tripled in the last two weeks.
The rapid growth of BA.4 and BA.5 in South Africa has drawn concern from health experts for a potential surge in the US.
"BA.4 and BA.5 are more contagious than BA.1 and BA.2, and have a better shot at evading existing immunity," Zhang Zuofeng, chair of the Department of Epidemiology at the University of California, Los Angeles, said in an interview with Xinhua on Tuesday.
Therefore, unvaccinated people who have been infected by Delta, and earlier Omicron strains like BA.1 or BA.2, have higher risk of get reinfected, Zhang said.
However, as the virus stays in the upper respiratory tract and causes less lung infection, it may have limited impact on severe diseases and deaths, Zhang told Xinhua.
Only a small number of cases of BA.4 and BA.5 have been documented in the US currently.
Zhang said the pair of new variants are likely to cause an increase in new infections in the US, but not like the outbreak caused by the original Omicron variant in January.
The US was now averaging about 60,000 new infections and 320 new deaths each day as of Monday, CDC data showed. The daily case count was up from roughly 25,000 infections per day in early April.