Science's COVID-19 reporting is supported by the Pulitzer Center and the Heising-Simons Foundation.

"We’re at risk of gambling away our success,” virologist Christian Drosten warned in the German newspaper Die Zeit last month. His message referred to Germany, but it could have been addressed to all of Europe. After beating back COVID-19 in the spring, most of Europe is seeing a resurgence. Spain is reporting close to 10,000 cases a day, more than it had at the height of the outbreak in the spring. France is back to reporting thousands of cases a day. In Germany, numbers are still low, but rising steadily. The pandemic is affecting countries that saw few cases in the spring, such as Greece and Malta, but is also rebounding in places that suffered terribly, including the cities of Madrid and Barcelona.

Drosten, of the Charité University Hospital in Berlin, is one of many calling for renewed vigilance, and he and others are urging a new control strategy that trades blanket lockdowns for measures specifically targeting clusters of cases, which play a key role in spreading the coronavirus. “We successfully aborted the [first] wave and now we should make sure that no new wave builds,” says epidemiologist Christian Althaus of the University of Bern.

The rising case numbers today aren’t quite comparable to the peak in April because countries are now testing far more people on a daily basis. But the increase shows that Europe relaxed measures too early and too much, says virologist Ab Osterhaus of the University of Veterinary Medicine in Hanover, Germany. “The wrong message was given, basically: We have done a great job and now we can relax again.” Instead, Europe could have tried to emulate New Zealand by stopping community transmission completely and zealously guarding against reintroductions, says Devi Sridhar, a global health expert at the University of Edinburgh who has been advising the Scottish government. Scotland committed early on to pushing case numbers down to zero, but other countries did not, and now almost all are seeing a resurgence.Few dispute that Europe rose to the initial challenge. In Bergamo, the capital of Italy’s Lombardy region, crematoria were so overburdened in March that army trucks had to transport the dead to other cities—but on 24 May, Lombardy registered zero COVID-19 deaths for the first time. By early July, the European Union and the United Kingdom together averaged fewer than 5000 new cases per day, whereas the United States and Brazil (which together have roughly the same population) had 50,000 and 40,000, respectively. Europeans enjoyed a surprisingly normal summer, with northern Europeans flocking to Mediterranean beaches.

People’s willingness to stay alert and remember new rules wanes quickly, says Cornelia Betsch, a psychologist at the University of Erfurt who has been monitoring attitudes toward the pandemic in Germany. “And we have been going for a while now, and the end is not even clear.” Some countries saw workplace infections rise as people returned to their offices, says Gianfranco Spiteri, a public health expert at the European Centre for Disease Prevention and Control. But in many countries the resurgence is driven by “young people partying and basically people living their life back in a kind of normal way,” he says. Because new cases are younger, fewer of them die, but “it’s a matter of time before the elderly are affected,” Spiteri says. The reopening of schools across the continent may make matters worse.

As in the spring, every country has its own strategies for controlling the pandemic, leading to a sometimes confusing patchwork. Belgium has one of the strictest face mask policies, for instance, but Belgians crossing the Dutch border to shop in Maastricht can take off their masks. Even within countries, the rules can change at dizzying speed. Germany went from a mandatory 14-day quarantine for people arriving from countries considered risky to voluntary tests at the airport and other entry points, with no quarantine for those who tested negative. Next, it made the tests mandatory, then returned to mandatory quarantine with testing after 5 days. “What would be necessary is that we define one central policy in Europe,” Osterhaus says. “The problem is, who is going to do that?” The European Union has little power to coordinate health measures.

Yet countries are better prepared this time. Whereas the virus spread largely under the radar in February, widespread testing now reveals its movements. (Fewer than 3% of tests are positive in most European countries, a sign of a healthy testing capacity.) Face masks, not available or even recommended in the beginning, have become ubiquitous in most countries. More than a dozen EU countries have developed apps to help contact tracing efforts. Better treatments are saving lives.

Meanwhile, new insights into viral spread are leading to better targeted control measures. The emphasis on hand hygiene is gone because it has become clear that contaminated surfaces don’t play a large role. In the spring, some countries banned almost any outdoor activity, including jogging; now, the focus is on indoor activities. “We’ve learned outdoor hospitality is generally fine, nonessential shops are fine as long as people wear face coverings, public transport doesn’t seem that risky,” Sridhar says.

Instead, public health experts increasingly argue for targeting clusters of cases and superspreading events. Some studies estimate that 10% of patients cause 80% of all infections, whereas most don’t infect anybody at all. Drosten has urged that contact tracers spend more time finding the source of a new case—along with that person’s contacts—than the new case’s contacts; after all, the patient may not infect anybody else, but is likely to have caught the virus as part of a cluster, Drosten says.

Adam Kucharski, a disease modeler at the London School of Hygiene & Tropical Medicine, agrees. “Looking backwards can actually give you a disproportionate benefit in terms of identifying infections,” he says. In a recent preprint, Kucharski and his colleagues estimated that “backward contact tracing” could prevent twice as many infections as tracing contacts forward alone. Experience in South Korea, where clusters at churches drove the epidemic early on, confirmed the value of this approach, says University of Florida biostatistician Natalie Dean.

Drosten also calls for a new approach in case health authorities are overwhelmed again: Only quarantining people who were in a potential superspreading situation with a newly identified case, but doing so immediately and then testing them after 5 days. That way, he says, contact tracers would spend their time in the best possible way and superspreading events could be quickly contained. (The public could help by making a short list every day of any potential cluster situations they were in.)

Putting more effort into finding clusters should also help epidemiologists understand where and how they emerge, says Hitoshi Oshitani of Tohoku University in Japan—which may have changed since the spring. “We’ve seen a massive change in the social structure and interactions of populations … from the start of the pandemic,” Kucharski says. The conditions that spread the virus then “won’t necessarily be the same ones that are creating the risk now.” In Germany, for instance, many large outbreaks early in the pandemic occurred in long-term care facilities. Now, clusters are increasingly reported from workplaces.

More-targeted measures probably won’t be enough to keep the virus from resurging, Althaus says. “A point will be reached again where stricter measures have to be taken,” he says. But rather than complete lockdowns, he assumes they will be more like the lighter version applied in Sweden, which encouraged people to work from home and banned large gatherings while keeping shops and restaurants open. Scotland recently closed pubs and restaurants in Aberdeen for more than 2 weeks after a cluster of cases emerged; it asked inhabitants not to travel more than 8 kilometers outside the city and visitors to stay away. But schools remained open.

Compared with the United States, Europe has one advantage as it faces its first pandemic winter: Control measures aren’t nearly as controversial. Protests against masks and social distancing broke out in many European cities in August, but they represented a small minority of the population, Betsch says. In Germany, support for control measures declined somewhat after infections peaked in spring, but a large majority still backs them, Betsch says. And with case numbers back on the rise, she says, “We can already see acceptance numbers go up again.”