The omicron subvariant BA.2, also known as “stealth omicron,” is now dominant in the United States.

Fifty-five percent of COVID-19 cases in the U.S. are now BA.2, according to data from the Centers for Disease Control and Prevention. That’s up from 39% last week, and up from 7.4% a month ago.

Other omicron subvariants — BA.1.1 and BA.1.1.529 — currently account for the the remaining 45% of infections. 

Health experts believe BA.2 is more contagious than previous omicron subvariants, but it does not appear more likely to cause severe disease. Vaccines, like with other versions of omicron, are less effective against BA.2 than they were against the original strain of the coronavirus or the alpha and delta variants. Booster shots, however, offer robust protection against hospitalization and death, studies show.

BA.2 has been called “stealth omicron” because it’s missing a gene found in BA.1 that makes it more difficult to track using a PCR test, meaning it can only be detected using genomic sequencing. 

While BA.2 has become more prevalent, the United States is experiencing some of its lowest case counts of the pandemic. The latest seven-day average for new COVID-19 infections is 27,775, down from more than 800,000 during omicron’s peak in January. 

But case numbers have leveled off over the past two weeks, and 13 states and the District of Columbia have seen increases during that time.

Dr. Anthony Fauci, the federal government’s top infectious disease expert, told ABC News last week that he would not be surprised if BA.2 causes an uptick in cases but that he doesn’t expect a surge.

A CDC survey in December found that about 95% of Americans 16 and older have COVID-19 antibodies from vaccination and/or infection.

Last week, the World Health Organization said BA.2 accounts for nearly 86% of all sequenced cases globally.