Rise of Omicron Subvariant in Europe Could Be Sign for U.S.

Experts warn that another coronavirus wave may be imminent in the United States, fueled by a more contagious Omicron subvariant that is spreading rapidly in Europe, though they said the trend was more a cause for caution than alarm.

The Omicron variant this month began its second sweep through Europe, where past virus surges have been a harbinger of what was to come in the United States. Many countries thought they were free of the worst of Covid and raced to lift restrictions in February and March, but a highly transmissible Omicron subvariant, BA.2, is contributing to the new surge.

The surges have not led to a widespread rise in hospitalizations in Europe, though the number of Covid patients is on the rise in a few countries, including Austria, Britain and the Netherlands, according to Our World in Data. And BA. 2 does not appear to cause more severe illness than BA.1, and existing vaccines are effective against it.

Some countries, including Germany and Austria, are again approaching record levels of caseloads or have even exceeded them. Cases per capita in Europe were already far higher than any other region in the world when they began creeping up again earlier this month. The continent is now recording 95 cases per 100,000 people, after bottoming out at 87 on March 3; the United States and Canada, by comparison, are recording 10 cases per 100,000 people, according to a New York Times database.

All 50 U.S. states made the same choice by the beginning of this month, lifting their universal indoor mask mandates after the winter Omicron surge retreated. Caseloads in the country have continued to drop since then, reaching levels this week not seen since last summer, according to the Times database.

But throughout the pandemic, Covid trends in the United States have lagged Europe’s by a few weeks. And U.S. wastewater data is already showing signs of a new increase.

Dr. Wafaa El-Sadr, a professor of epidemiology and medicine at Columbia University, said that the data was a sign that people should be cautious.

“But I would not say that people should be alarmed as of yet,” she said on Wednesday.

Because people excrete the virus through their stool, wastewater can be used to predict where the coronavirus is or will be prevalent and if a new variant is circulating.

About 38 percent of active U.S. wastewater sampling sites reported an increase in coronavirus levels from Feb. 26 to March 12, according to the Centers for Disease Control and Prevention’s wastewater data tracker, which surveys 698 wastewater sites across the country.

The BA.1 subvariant remains the predominant Omicron subvariant in the United States, but BA.2 is more contagious and spreading fast, so Dr. El-Sadr said it was important for Americans to be vaccinated and boosted. “This will protect us from what’s coming next,” she said.

BA.2 is responsible for only a fraction of coronavirus cases in the United States, but that share is growing, according to estimates from the C.D.C. The subvariant’s spread in the United States depends on a number of factors, including people’s existing immunity and the subvariant’s severity.

Dr. Maria Van Kerkhove, the World Health Organization’s technical lead for coronavirus response, said that BA.2 was one of the factors driving the recent global uptick in cases, fueled by high caseloads in Asia and Europe. Other factors include lifting pandemic restrictions, vaccine resistance and misinformation, she said.

“We have huge amounts of misinformation that’s out there,” she said at a news briefing on Wednesday. “The misinformation that Omicron is mild, misinformation that the pandemic is over, misinformation that this is the last variant that we will have to deal with.”

Sophie Downes contributed reporting.