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BERLIN — Several European countries that had their coronavirus outbreaks under control have begun to see a rise in cases that is feeding fears of a second wave.

Governments are urging their citizens to be more vigilant amid the lure of summer gatherings and vacations, while health officials warn that lax public attitudes are putting the continent on a dangerous trajectory.

A spike in infections has led Belgium to ramp up restrictions on social contact, while Spain has closed gyms and nightclubs in Barcelona.

Meanwhile, German health officials have called a rise in infections in the past two weeks deeply concerning.

“People are being infected everywhere,” said Ute Rexroth, head of surveillance at Germany’s Robert Koch Institute, which sounded the alarm on rising numbers on Tuesday. “Weddings, meetings with friends, sadly, also nursing homes or health institutes. We are worried that this could be a change of trend.”

Almost 40 countries have reported record single-day increases in coronavirus infections showing a resurgence in every region of the world. (Reuters)

The rise in cases across several countries follows weeks of stability that had ushered in a growing sense of normalcy. A wave of reopening measures had come and gone without significant ill effect. People went to movies, dined at restaurants and started working from offices again.

But some virologists had warned that openings would inevitably be followed by new infections. Others stressed that successful reopening was dependent on citizens wearing masks and maintaining social distance. And there are signs adherence has been slipping.

On the tree-lined streets of Brussels, masks have been a rare sight. In Berlin, famed for its 24-hour pre-pandemic party scene, police have struggled to break up crowds of weekend revelers who gather in parks and open spaces for illicit dance parties. Spanish nightclubs and beaches brimmed with vacationers after European travel restrictions eased.

“More and more people are getting relaxed,” said Dirk Brockmann, a professor at Berlin’s Humboldt University who works on pandemic modeling with the Robert Koch Institute. “People are wearing a face mask going shopping or on the subway, but other than that they are going back to normal.”

He said that if the rise in infections was linked to reopenings rather than subsequent relaxed behavior, there would have been a stronger indication earlier.

What happens now may similarly depend on human behavior, he added.

“Potentially people will change their behavior again and we will mitigate,” he said.

The number of new cases is still far below what European countries saw during the peak of their outbreaks — or what is happening in the United States.

Germany, for instance, recorded 633 new cases on Tuesday, compared with more than 6,000 daily cases at its peak. But German health authorities said the nature of the new infections is concerning, with outbreaks no longer largely confined to slaughterhouses or nursing homes.

“Corona is coming back with all its might,” warned Bavarian state premier Markus Söder, according to German news site Merkur.

In France, new cases hit an average of 850 over the past three days — nowhere near the average of 2,582 in April. But the Health Ministry noted that recent progress has been erased, and Health Minister Olivier Véran warned of as many as 500 active clusters.

In Belgium, cases suddenly started rising this month after declining consistently since April. There were 707 cases diagnosed nationwide the week of July 6. Just two weeks later, the figure had tripled to a level not seen since early May.

Belgian policymakers are tightening restrictions. Customers are required to wear masks in stores. Starting Wednesday, Belgian households will be asked to limit consistent close social contact to no more than five people, down from 15. The maximum size of social gatherings, such as weddings, has been slashed to 10 people, down from 50. A country that had insouciantly planned to reopen schools without significant restrictions in September is now questioning whether that will happen.

“What is happening is a situation that is worrying, not terrifying,” Belgian Prime Minister Sophie Wilmès told Le Soir newspaper. However, she has warned that a second lockdown may be necessary.

There’s debate about whether Europe might be at the beginning of a second wave. British Prime Minister Boris Johnson suggested as much on Tuesday.

“Let’s be absolutely clear about what’s happening in Europe, amongst some of our European friends. I’m afraid you are starting to see in some places the signs of a second wave of the pandemic,” he said.

Johnson was defending an advisory against nonessential travel to Spain, where the region of Catalonia has reemerged as a hot spot, with thousands of new cases reported in the past two weeks. Spanish Health Minister Salvador Illa has largely linked new cases to seasonal farmworkers, people attending family get-togethers and nightclubs.

“We aren’t at the beginning of a second wave,” said Pamela Vallely, a medical virologist from Manchester University. “This is just the continuation of the virus being in our communities.”

Resurgences are “just what’s going to happen,” she said. “We are going to have to continue to try to contain things to levels we can cope with.”

But as governments now have experience with how to cope with the virus, including tools such as localized lockdowns, testing and contact tracing, she said she doesn’t envisage a large surge.

“I’m relatively optimistic,” she said.

Birnbaum reported from Riga, Latvia, and Weber-Steinhaus from Hamburg. James McAuley in Paris contributed to this report.