The US is trudging into what could be another COVID-19 surge, with cases rising nationally and in most states after a two-month decline.

“We don’t know how high that mountain’s gonna grow,” said Dr Stuart Campbell Ray, an infectious disease expert at Johns Hopkins University.

No one expects a peak nearly as high as the last one, when the contagious omicron version of the coronavirus ripped through the population.

But experts warn that the coming wave – caused by a mutant called BA.2 that’s thought to be about 30 per cent more contagious – will wash across the nation. They worry that hospitalisations, which are already ticking up in some parts of the Northeast, will rise in a growing number of states in the coming weeks. And the case wave will be bigger than it looks, they say, because reported numbers are vast undercounts as more people test at home without reporting their infections or skip testing altogether.

At the height of the previous omicron surge, reported daily cases reached into the hundreds of thousands. As of Thursday, the seven-day rolling average for daily new cases rose to 39,521, up from 30,724 two weeks earlier, according to data from Johns Hopkins collected by The Associated Press.

Dr Eric Topol, head of Scripps Research Translational Institute, said the numbers will likely keep growing until the surge reaches about a quarter the height of the last “monstrous” one. BA.2 may well have the same effect in the US as it did in Israel, where it created a “bump” in the chart measuring cases, he said.

Keeping the surge somewhat in check, experts said, is a higher level of immunity in the US from vaccination or past infection compared with early winter.

But Ray said the US could wind up looking like Europe, where the BA.2 surge was “substantial” in some places that had comparable levels of immunity. “We could have a substantial surge here,” he said.

Both experts said BA.2 will move through the country gradually. The Northeast has been hit hardest so far — with more than 90 per cent of new infections caused by BA.2 last week compared with 86 per cent nationally. As of Thursday, the highest rates of new COVID cases per capita over the past 14 days were in Vermont, Rhode Island, Alaska, New York and Massachusetts. In Washington, D.C., which also ranks in the top 10 for rates of new cases, Howard University announced it was moving most undergraduate classes online for the rest of the semester because of “a significant increase in COVID-19 positivity” in the district and on campus.

Some states, such as Rhode Island and New Hampshire, saw the average of daily new cases rise by more than 100 per cent in two weeks, according to Johns Hopkins data.

Data from the Centers for Disease Control and Prevention shows that new hospital admissions of patients with confirmed COVID-19 were up slightly in New England and the New York region.

Ray said government leaders must be careful to strike the right tone when talking to people about protecting themselves and others after COVID restrictions have largely been lifted.

“It’s going to be hard to institute restrictive, draconian measures,” Ray said. “Fortunately, we have some tools that we can use to mitigate risk. And so I hope that leaders will emphasise the importance for people to watch the numbers,” be aware of risks and consider taking precautions such as wearing masks and getting vaccinated and boosted if they’re not already.

Vigilance is a good strategy, experts said, because the coronavirus is constantly throwing curveballs. One of the latest: even more contagious subvariants of BA.2 found in New York state, known as BA.2.12 and BA.2.12.1. And scientists warn that new and potentially dangerous variants could arise at any time.

“We shouldn’t be thinking the pandemic is over,” Topol said. “We should still keep our guard up.”