The Omicron subvariant BA.2 now accounts for ‘around 30 percent’ of New York City’s cases, officials say.

An even more transmissible version of the Omicron variant of the coronavirus known as BA.2 now accounts for about 30 percent of new cases in New York City, health officials said on Friday.

The subvariant is more transmissible than BA.1, the Omicron version that drove a sudden spike in cases and hospitalizations in New York last December and disrupted the city’s path to economic recovery.

Dr. Ashwin Vasan, who started this week as the city’s new health commissioner, said that he and others were watching closely to see if New York was on the verge of experiencing a wave of BA.2 cases like those taking place in parts of Europe and Asia, including Hong Kong. But as of this week, he said, there is no cause for immediate alarm.

“Community spread remains low,” Dr. Vasan said, in his first news conference as health commissioner. “Hospitalizations and deaths are stable or decreasing.”

New cases of the virus had been declining sharply since early January, when the surge that started the month before began to plummet from a peak of more than 40,000 average daily cases, leaving a relatively low death toll. But cases have been increasing in recent days, said Dr. Celia Quinn, the deputy commissioner who oversees the Health Department’s public health laboratory.

As of Thursday, an average of 905 cases per day were reported in New York City, a 35 percent increase from the average of less than 700 two weeks ago, according to a New York Times database. The uptick comes as Mayor Eric Adams has rolled back some mask and vaccination requirements as part of a bid to restore a sense of normalcy in the city and promote economic recovery.

Officials will continue to monitor cases and hospitalization rates, Dr. Quinn said. But she noted that BA.2 “does not appear to cause more severe illness” than BA.1, and she and Dr. Vasan said they believed hospitals would be prepared if there were any significant increase in severe illness.

BA.1 is responsible for the majority of cases in the United States, but the share of BA.2 cases is growing, reaching about 23 percent, according to estimates published Friday by the Centers for Disease Control and Prevention.

Epidemiologists are not yet sure whether BA.2 will drive a new spike in virus cases and related hospitalizations. BA.1 proved to be intrinsically less severe than prior variants, causing less damage in the lungs, and researchers in Britain and Denmark have found that BA.2 does not carry a higher risk of hospitalization. Vaccines were also effective at reducing severe illness from BA.1, particularly among those who received a booster shot, and researchers in Britain and Qatar have found the same for BA.2.

But though 77.4 percent of New York City residents are fully vaccinated, just 36.1 percent have received an additional vaccine dose.

“It’s essential that New Yorkers go and get boosted when they’re eligible,” Dr. Vasan said.

In New Jersey, Gov. Philip D. Murphy said that he and the state’s health officials expect to “eventually see an increase in the number of cases” that follows their rise across the globe. The state’s seven-day average for confirmed cases was 790 on Friday, down 12 percent from a week ago, Mr. Murphy said.