U.S. COVID rates are currently on the decline, but experts say that a virus surge in Europe means that Americans should expect their own wave soon.

“The next wave in Europe has begun,” Eric Topol, founder and director of the medical research Scripps Institute, wrote on Twitter on March 12, sharing a chart showing rapid upticks in the COVID-19 case counts of Austria, the Netherlands, Germany, Greece, and more.

For almost the entire pandemic, the U.S. has closely followed trends happening in Europe. Each time, a surge of virus caseloads in Europe forecast a similar outbreak in the U.S. weeks later.

“The U.S. typically follows Europe by about two to four weeks. We've seen this throughout the pandemic, and it's very likely that we're going to see a spike in the U.S. over the next probably one to three weeks,” John Wherry, director of the University of Pennsylvania’s Institute for Immunology, told Fortune.

Experts say that the current wave sweeping Europe comes down to a few big factors, including the spread of Omicron’s subvariant (dubbed stealth Omicron), governments relaxing restrictions, and people changing their behaviors and conception of the virus.

Those factors are now taking hold in the U.S., as most jurisdictions have dropped their mask mandates and distancing requirements, and people desperately want to return to normal.

“Those variables are relevant in the U.S. and lead me to believe that a similar surge in cases is likely to occur here,” Dr. Josh Schiffer, infectious disease researcher at the Fred Hutchinson Cancer Research Center, told Fortune. “It seems likely that we're at risk for a similar situation as is occurring in Europe right now.”

“Stealth Omicron”

One of the biggest sources of new infections in Europe over the past few days has been the rise of a new form of the virus, the BA.2 variant, colloquially known as “stealth Omicron.”

It was first identified in Europe in January and has spread throughout the continent quickly. The difference between stealth Omicron and regular Omicron (BA.1), which has been circulating in Europe since November, is that the former appears to be even more contagious and harder to detect.

“It's dominant here across the U.K., and it’s supposed to be significantly more transmissible than BA.1,” Christian Yates, lecturer in mathematical biology at the University of Bath in England, told Fortune. “So I think that could be leading to the big rises that we're seeing across the U.K. and Europe.”

New COVID cases in Britain have been on a steep rise since the end of February, as over 500,000 people tested positive for the virus in the last week. Stealth Omicron accounts for around 57% of new cases.

Yates says hospitalization rates in the U.K. are still nowhere near the all-time highs seen in earlier phases of the pandemic, but he said he thinks the rapid spread of stealth Omicron could change that. In the week ending March 12, more than 11,000 patients with COVID were admitted to hospitals, according to the National Health Service, a 21% increase from the previous week.

Stealth Omicron infection rates in the U.S. are much lower than those in Europe, but rising fast. By the end of February, the subvariant accounted for around 8% of infections in the U.S., but that share is now up to nearly 25%.

“What I can say confidently is that [stealth Omicron] will become the predominant variant in the U.S. eventually, based on its ability to outcompete the original Omicron,” Schiffer said.

Relaxed restrictions

Around the time of the original Omicron surge in Europe, governments worldwide—encouraged by the seemingly lower severity of the new variant—began to gradually lift more pandemic restrictions.

Europe experienced its Omicron wave in December and early January, and countries began relaxing restrictions in January and February. Italian authorities issued policies in the first week of January aimed at keeping schools open despite rising caseloads. The U.K. began easing mask mandates and COVID pass requirements starting at the end of January, and Denmark abandoned all virus restrictions at the beginning of February.

Experts believe this relaxation of restrictions may have played a role in changing behaviors, which in turn is fueling the current surge.

“The government messaging is that the COVID pandemic is over in the U.K.,” Yates said. COVID-19 is no longer discussed as a threat in the country, leading many to behave differently than they have during other surges.

In the U.S., some states have not had a mask mandate in place since last spring, but many have dropped their COVID restrictions over the past few months. Hawaii is the last holdout, and will stop enforcing masks later this month.

Other major global news stories have also redirected more people’s attention away from the main pandemic story that has been dominating the news cycle for two years. For many people, whether they had COVID or not, restrictions and constant coverage were often a more palpable reminder that we are living in a pandemic than the virus that is causing it.

”COVID has dropped out as a news headline here, and understandably so because of the horrific crisis in Ukraine,” Yates said.

Why next COVID surge could be less disruptive

But even if COVID cases in Europe continue to rise, and a new wave of stealth Omicron overwhelms the U.S., experts say it’s unlikely to be as disruptive as previous surges.

The strong vaccination campaigns carried out in Western Europe and the U.S. have left behind more robust immunity among the general populations. That means that people will still get infected, but it’s likely that fewer will end up in the hospital.

Around 77% of the U.S. population has received at least one vaccine dose, 65% is fully vaccinated, and 29% have received a booster, according to data from the CDC.

“We have a high level of underlying immunity in [the U.S.] at the moment, particularly among the highest risk group, which is the elderly, where we really have done a nice job in terms of vaccinating that population,” Schiffer said.

Some countries and regions that have not focused so heavily on vaccinations are currently experiencing very different types of surges. Hong Kong is grappling with its worst wave of infections yet, but with less than half of the city’s 1 million residents over 70 vaccinated before the current outbreak, the city’s death rate from COVID-19 registered as the highest in the world last week.

“What's happening in Hong Kong right now is very much akin to what happened in Italy, in Spain, and in parts of the U.S. during the early 2020s,” Schiffer said.

And even though a stealth Omicron surge in the U.S. is less likely to be as severe as previous waves because of increased immunity, the virus will continue to pose a threat to millions of vulnerable people. The risk of long COVID is also an ongoing mystery and a danger to anyone who catches COVID. And for as long as the virus is still circulating, new waves will continue to emerge.

COVID falling out of the news cycle is natural, but it may breed complacency, according to Wherry. “That's true for individuals, but also policymakers, politicians, and public health officials. We're not out of the woods,” he said.

In the U.K., that seems to already be happening.

“Many people in the U.K. are not even aware that there's a surge going on at the moment,” Yates said. “People are not having the idea that COVID is a threat reinforced anymore, and so perhaps not protecting each other in the same way.”