The Omicron BA.2 subvariant, which is believed to be one of the drivers of a recent COVID-19 surge in Europe, now accounts for 84.3 percent of new cases in New England, according to estimates from the CDC.
Nationally, BA.2 has grown to account for 72.2 percent of cases, up from 1 percent as recently as early February, according to the CDC.
The increases come as experts and officials warily eye the possible impact of the more-infectious BA.2, which is arriving just as a pandemic-weary nation was recovering from the considerable original surge of Omicron, which peaked early this year.
“It’s certainly concerning. We don’t know quite what path things are going to take in the coming weeks, but we know BA.2 is more transmissible, and we certainly know there are many folks who are not boosted or not vaccinated who might be susceptible to exposure,” said Jonathan Levy, who chairs the department of environmental health at BU’s School of Public Health.
Massachusetts has seen a small bounce upward from low levels in metrics such as statewide case numbers and the amount of coronavirus detected in wastewater from Eastern Massachusetts communities. The percentage of positive tests in Boston has also ticked upward.
The resurgence in Europe has been blamed on factors including BA.2, the loosening of pandemic restrictions, and the waning of immunity from vaccines or prior infections.
Experts are concerned the same factors are in play in the United States. But they have also suggested there might be key differences between the European and US situations. One difference they cite is the greater proportion of people here who have already been infected by Omicron and developed some immunity to BA.2.
Another possible difference raised by experts: BA.2 is arriving a little bit later in the United States than in Europe, at a time of year, spring, that seems less favorable to the coronavirus.
A spokeswoman for the Massachusetts Executive Office of Health and Human Services said in an e-mail last week the state was working with the health care community and the federal government “to monitor all of the latest developments.” She emphasized that Massachusetts is a national leader in vaccination rates and said people had “ready access to vaccines, rapid tests, and therapeutics.”
State officials say residents should get fully vaccinated and boosted to protect themselves from the virus, get tested if they need it, and if they get a positive result, talk to their doctor right away about COVID-19 treatments available for individuals who have mild to moderate symptoms.
Material from Globe wire services and prior Globe stories was used in this report.