In this Feb. 9, 2022, file photo, passers-by wear masks under their chins as they chat with one another while crossing a street in Boston. Credit: Steven Senne / AP

Another omicron subvariant that’s apparently “even more transmissible” than the last one is gaining steam around the region, as COVID-19 case counts continue to rise.

The subvariant BA.2.12.1 — an offshoot of the BA.2 omicron “stealth” variant — now accounts for 20 percent of new COVID-19 cases in New England, according to the CDC tracker. That’s almost double from 11.5 percent during the previous week.

New York health officials recently sounded the alarm about the emergence of subvariants BA.2.12.1 and BA.2.12, warning that these highly contagious new variants are likely contributing to rising cases. The subvariants have been estimated to have a 23 percent to 27 percent growth advantage above the original BA.2 variant.

“Another new strain that appears to be even more transmissible than the last and that would explain at least some of the rise in cases we are starting to see, though I think we would have seen one even without this,” said Matthew Fox, a Boston University School of Public Health epidemiology professor.

“The good news is that it does not appear to be more severe than any of the last omicron variants — at least so far — and so we are really looking at more cases, but hopefully not a sharp increase in hospitalizations and deaths,” he added. “We will have to keep our eye on it.”

A lot is still not known about BA.2.12.1, added Davidson Hamer, a Boston University School of Public Health infectious diseases specialist. There have not been sufficient studies to adequately characterize its transmissibility, he said.

“Since there is evidence that BA.2.12 and BA.2.12.1 are quickly becoming the major sublineages of BA.2 in New York and New England, this suggests that it may be more transmissible as it is displacing other omicron-related variants,” Hamer said.

“Similarly, there are not any solid data yet on its severity, so it is hard to say much about this,” he added. “We certainly need to track this closely and try to learn more about it over the next few weeks.”

Virus cases had been plummeting in Massachusetts following the omicron surge but then started rising several weeks ago as the BA.2 omicron “stealth” variant took hold. The daily average for cases in the Bay State has jumped from about 600 infections in mid-March to nearly 2,000 cases last week.

Meanwhile, mask mandates have been lifted, including the mask order on public transportation.

“I am concerned about the worsening wave in Massachusetts,” Hamer said. “It is not as abrupt as omicron was, but it has continued to progress based on the wastewater data, rising case counts and increasing proportion of positive tests.”

Story by Rick Sobey, Boston Herald.