Cases of a highly transmissible Omicron subvariant were doubling in the United States each week over the past month, according to the data updated Tuesday by the U.S. Centers for Disease Control and Prevention (CDC).
Health experts are keeping close watch on the variant, known as BA.2, urging the public to remain cautious and not let down guard against COVID-19.
BA.2 accounted for only 1 percent of new cases in the week ending Feb. 5, rising to 2.2 percent in the week ending Feb. 12, and 3.8 percent in the week ending Feb. 19, and 6.6 percent in the week ending Feb. 26, CDC data showed.
The variant was responsible for 11.6 percent of coronavirus infections in the country in the week ending March 5, according to the CDC estimates.
BA.2 has slowly, but steadily spread even as the Omicron surge continued to decline in the United States.
New lab and animal experiments suggested that BA.2 may be capable of causing more severe disease than the original Omicron strain.
Early studies showed that BA.2 may be up to 30 percent more transmissible than the original Omicron.
Health experts said that without mitigation measures such as mask-wearing, hand washing, and social distancing, the latest version of Omicron can spread rapidly in communities.
Some experts said it is unlikely BA.2 will trigger a massive new surge because the majority of people in the country now have immunity from prior infections or vaccination against COVID-19.
"The most likely thing that's going to happen is that it might extend our tail, meaning it might slow down the decrease in cases. But it's probably not going to lead to a new wave of cases," said Nathan Grubaugh, an associate professor of epidemiology at the Yale School of Public Health.
Adding to the concern, one of the remaining antibody treatments for COVID-19 may be less effective against BA.2, according to recent research.
Jeremy Luban, a virologist at the University of Massachusetts Medical School, told the National Public Radio that there are going to be plenty of people getting sick and ending up on respirators and dying because of BA.2, especially among the millions who have not been vaccinated.
CDC Director Rochelle Walensky warned Americans not to let down their guard entirely as more states have lifted mask and vaccination mandates.
"We want to give people a break from things like masking when our (COVID-19 community) levels are low, and then have the ability to reach for them again should things get worse in the future," she said.
"We need to be prepared, and we need to be ready for whatever comes next," Walensky said.
The country is now averaging about 40,000 new cases and 1,200 new deaths daily, according to the latest CDC data.