The Omicron subvariant BA.2 is now the dominant strain of the coronavirus in the US, accounting for nearly 55 percent of all new infections last week, the Centers for Disease Control and Prevention (CDC) said Tuesday.

Scientists believe BA.2 is more transmissible than the original Omicron strain, though it isn't believed to cause more severe COVID-19. 

The hardest-hit region is the Northeast, where the variant represents more than 70 percent of sequenced cases in New York and New Jersey, according to the CDC.

The South and Mountain West are seeing the fewest cases, where BA.2 is causing slightly more than one-third of infections.

BA.2 has doubled as a proportion of the circulating variants of the virus in the US about every two weeks. At the beginning of February, the Omicron subvariant made up just 1 percent of sequenced coronavirus cases in the US.

Nationally, the seven-day average of newly reported cases remained around 30,000 for about two weeks, according to Johns Hopkins University data.

The signs of a resurgence came after dozens of states have moved to close public testing sites. Case data have become less reliable with more people using at-home COVID-19 tests, whose results generally are not reported to officials. Experts said infection totals are likely significantly undercounted. 

"We certainly will be seeing increase in cases," Dr Anthony Fauci, chief White House medical adviser, said during an interview with the BBC this week, warning that it may be necessary to adopt some mitigation and masking measures should the US see a resurgence in hospitalizations.

"We need to be prepared for the possibility that would have another variant that would come along," Fauci said. "If things change, and we do get a variant that does give us an uptick in cases of hospitalization, we should be prepared and flexible enough to pivot towards going back at least temporarily to a more rigid type of restrictions such as requiring masks indoors."

The Food and Drug Administration (FDA) on Tuesday authorized second booster shots of the Pfizer-BioNTech and Moderna COVID-19 vaccines for people 50 and older. The FDA decision opens a fourth dose of the Pfizer or Moderna vaccines to that group. The agency said those who are eligible can get the additional shot at least four months after their previous booster.

The FDA also authorized a second booster for people 12 and older who have a severely weakened immune system, with the same interval between boosters.

The decision means that tens of millions of Americans are now eligible for another shot.

The CDC is expected to issue advice on who should consider getting the shot. 

But the second booster is only for the higher-risk groups, FDA vaccine chief Peter Marks said, because there is evidence protection can wane and the FDA decided the option "will help save lives and prevent severe outcomes".

There is limited evidence of how much benefit another booster could offer right now. The FDA made the decision without input from its independent panel of experts, which has been grappling with how much data is needed to expand shots, The Associated Press reported. 

"At some point we're going to have to realize that this is a virus that's going to be with us and that we have to come to grips with dealing with it on a regular basis," Marks said.