The new strain, called BA.2.12.1, made up 36.5 percent of new COVID-19 cases in the country in the week ending April 30, according to data released Tuesday by the U.S. Centers for Disease Control and Prevention (CDC).
The data increased from 26.6 percent a week before, and 16.7 percent two weeks prior, CDC data showed.
The majority of COVID-19 cases in the United States - around 61 percent - are still caused by BA.2., which has become the country's dominant variant since March.
Some regions, like the Northeast, are seeing more cases caused by BA.2.12.1 than others. The New York State Department of Health first announced the emergence of the subvariant in mid April. It was responsible for 41.6 percent of infections across the state as of April 23, according to the latest data of the department.
"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 percent 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 United States.
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