The BA.2 Omicron subvariant of the coronavirus is now responsible for 86% of US COVID-19 cases and more than 90% of infections in the Northeast, according to data on Tuesday from the US Centers for Disease Control and Prevention (CDC).
COVID-19 infections have been back on the rise during the last few weeks, particularly in Northeast states such as New York, Connecticut and Massachusetts, although overall cases have dropped sharply nationally since hitting record levels in January, according to data from the agency.
A resurgence in COVID-19 cases in parts of Asia and Europe has raised concerns that another wave could follow in the United States, as has been the case with previous surges during the pandemic.
Daniel Kuritzkes, chief of division of infectious diseases at Boston's Brigham and Women's hospital, said the trend of an overall decline in cases has now reversed.
"It's clear that there is now a trend towards an increasing number of cases across the country and particularly in the Northeast," Kuritzkes said.
The seven-day moving average of US COVID-19 cases stood at 28,339 as of April 9, up nearly 10% from a week earlier, the CDC said, with BA.2 accounting for 85.9% of infections.
For the week ending April 2, BA.2 made up 75.4% of the variants identified in the country, up from a previous estimate of 72.2%, according to CDC estimates.
Philadelphia, in the northeastern state of Pennsylvania, will reimpose indoor mask mandate from April 18, as a response to a fresh wave of cases, making it the first major U.S. city to take that step.
"I think it's likely that if the numbers pick up in the same way, individual cities will reimpose masking requirements," said Kuritzkes. "I don't think it's likely that will happen at the state level."