LONDON — For all of the challenges in controlling the spread of the coronavirus, Europe’s initial strategy was relatively straightforward: nearly universal, strictly enforced lockdowns.
It eventually worked. And in the two months since most countries have opened up, improved testing and tracing have largely kept new outbreaks in check. With basic rules on wearing masks and social distancing, life has been able to resume with some semblance of normality.
But in recent days France, Germany and Italy have experienced their highest daily case counts since the spring, and Spain finds itself in the midst of a major outbreak. Government authorities and public health officials are warning that the continent is entering a new phase in the pandemic.
There isn’t the widespread chaos and general sense of crisis seen in March and April. And newly detected infections per 100,000 people across Europe are still only about one-fifth the number in the United States over the last week, according to a New York Times database.
But there are growing concerns that with the summer travel season drawing to a close, the virus could find a new foothold as people move their lives indoors and the fall flu season begins.
With countries employing a variety of strategies — and with rules often changing suddenly and guidance varying from nation to nation — it remains to be seen which tactics will prove both enforceable and effective.
The virus is also spreading across a landscape vastly changed from the one it found in the spring, with many cities’ centers still largely empty of office workers and a public on guard.
The increase in cases in Europe, as in many other parts of the world, is being driven by young people. The proportion of people age 15 to 24 who are infected in Europe has risen from around 4.5 percent to 15 percent in the last five months, according to the World Health Organization.
Dr. Hans Kluge, its director for Europe, said on Thursday that he was “very concerned” that people under age 24 were regularly appearing among new cases.
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“Low risk does not mean no risk,” he said. “No one is invincible, and if you do not die from Covid, it may stick to your body like a tornado with a long tail.”
This time, European leaders have largely avoided imposing widespread lockdowns, and are instead relying on measures like targeted restrictions on movement in hot spots, increased mask requirements and public health education campaigns.
Dr. Kluge said that strategy could work.
“Between the basic measures at the national level and additional targeted measures, we are in a much better position to eradicate localized viral outbreaks,” he said. “We can manage the virus and keep the economy and the education system running.”
In France, President Emmanuel Macron has ruled out another countrywide lockdown, opting instead for “very localized strategies.”
“We cannot bring the country to a halt, because the collateral damage of confinement is considerable,” he told Paris Match magazine this week, adding that “zero risk never exists in a society.”
A growing number of French cities have made mask wearing mandatory in crowded streets and markets, and on Thursday the southern cities of Nice and Toulouse became the first to extend the rule to all outdoor areas.
But as the number of new infections rises daily — on Thursday, there were nearly 4,800 new infections, a figure not reached since April — some wonder whether the government is being too lax.
Olivier Véran, the health minister, acknowledged on Friday that the virus’s spread was “accelerating,” but he said the situation would remain under control as long as people observed social distancing and hygiene measures.
“We are several days away from the return from vacations,” Mr. Véran said, warning that “people are going to get back to their lives” in places like offices and schools.
“The virus must not spread from younger people to older people,” he added.
A surge in cases in Spain, however, illustrates the difficulties of an ad hoc approach to virus suppression.
Since the lifting of a state of emergency in June, 17 regional governments in the country have been directing their own efforts. That has left Spain splintered into a mosaic of different rules, many of which had to be changed almost immediately once hundreds of local outbreaks were identified.
Countries like Britain have now introduced self-quarantine rules on travelers coming from Spain, wrecking Spanish hopes of a strong summer tourism recovery. Nightclubs were closed again within weeks of reopening, and some Spanish regions have recently gone further, including banning smoking in public outdoor spaces.
The back-and-forth has also been coupled with uncertainty over whether regions are doing enough testing and tracing of infections. In the Madrid region, labor unions representing schoolteachers voted on Wednesday to go on strike rather than reopen classrooms in September, in protest over what they consider inadequate safety guarantees from the regional government.
“Nobody should be in any doubt,” Dr. Fernando Simón, the director of Spain’s health emergency center, said on Thursday. “Things are not going well.”
The approach in Britain, which has had the greatest rate of excess deaths in Europe during the pandemic, has a similarly disjointed feel, with sudden rule changes often confusing the public.
In Birmingham, residents are facing the return of the “dark days” of lockdown, a local official has said, after a surge of new cases was reported. In northern England, including around Manchester, people from different households have been barred from meeting.
But the country’s health secretary, Matt Hancock, told the BBC this week that workers should go back to their offices. And the government is funding an initiative to get people back out into restaurants, covering a portion of the cost of some meals.
The authorities are also requiring 14 days of self-quarantine for travelers coming from Austria, Croatia, France and the Netherlands, and have warned that more countries could be added.
In Germany, Chancellor Angela Merkel has vowed to tackle the spread of the virus without closing national borders, despite a rise in daily infections not experienced since the end of April. She said on Thursday that the European Union must be united and that it needed to “act even more European” to stop the virus.
“I don’t think we’re just going to close the borders again,” she said. “Politically, we really want to avoid that at all costs.”
Nearly 40 percent of recent new infections in Germany have been brought back by returning vacationers, according to the government.
The country’s Foreign Ministry has warned against traveling to several popular destinations, including most of Spain and parts of Croatia. But returning travelers can now be tested for free at German airports.
And even if political leaders did want to impose lockdowns again, there are indications that the public would not be so compliant a second time around.
This month, tens of thousands of people in Berlin took part in demonstrations against coronavirus restrictions. And in the Netherlands, dozens of protesters from a group claiming that the virus was a government hoax clashed with police officers in The Hague on Thursday, an extreme example of rising tensions over the Dutch government’s handling of the pandemic.
Face masks, now common in much of Europe, have been a subject of confusion in the Netherlands, where the head of the country’s National Institute of Public Health, Jaap van Dissel, has said that masks offer “fake protection.”
Although masks are now required on public transit, the Dutch government says it is more important for people to stay six feet apart in all situations. It has also urged people not to have more than six guests in their homes.
Public wariness with regulations has been blamed for an uptick of cases in Belgium.
As the summer has progressed, many people there stopped wearing masks in stores, and the police have had to break up partying students at major squares in Brussels.
As the infection rate has ticked up, politicians issued more restrictions and Prime Minister Sophie Wilmès issued broad mask-wearing requirements.
“The future will depend on the behavior of everyone,” she said. “These are not suggestions, but orders.”
Reporting was contributed by Allison McCann, Elian Peltier and Kaly Soto from London; Raphael Minder from Madrid; Christopher F. Schuetze from Berlin; Thomas Erdbrink from Amsterdam; Elisabetta Povoledo from Rome; Aurelien Breeden from Paris; and Julia Echikson from Brussels.