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(CNN) - For months, respiratory viruses like RSV and the flu have had a grip on the country.

As cases of those viruses seem to be trending down, COVID-19 is once again showing signs of a winter surge.

”This is a new emerging subvariant and it has risen very quickly across the country,” said White House COVID-19 Response Coordinator Dr. Ashish Jha.

One in five Americans now live in a county that the Centers for Disease Control and Prevention considers to have a high community level of COVID-19.

Only about 15% of those eligible for an updated booster have gotten it, and about 20% of people in the U.S. are completely unvaccinated against COVID-19.

”People who have gotten that updated bivalent booster, all the evidence so far suggests that they’re still protected against even this XBB.1.5,” Jha said.

The omicron subvariant has quickly taken hold in the Northeast, and health experts are watching to see if it will spread to other parts of the country, too.

With at-home rapid tests and cases not being reported, the exact COVID-19 burden on the U.S. is still unclear. Some data suggest viral levels in wastewater are increasing.

Hospitalization numbers have now surpassed last summer’s BA.5 wave, and seniors are being hit particularly hard.

”Obviously right now, COVID-19 is the thing that’s increasing that we need to pay the most attention to,” Jha said.

The latest omicron subvariant is estimated to be the cause of nearly 60% of all new COVID-19 infections in the Northeast. Hospitalizations also are rising there.

It is not clear yet whether XBB.1.5 causes more severe disease than some recent subvariants or if waning immunity is leaving people less protected from COVID-19 infections.