The coming months will test whether high vaccination rates and a patchwork of low-intensity public health measures can keep the highly contagious Delta variant in check as temperatures drop and people huddle indoors.

A season free of another calamitous outbreak will demonstrate that daily life can move much closer to pre-pandemic normalcy thanks to the protection of vaccines. But another burst of severe illness and death will show the virus can still find gaps in our immunological armory and will take longer to suppress.

Even with significant majorities of their populations vaccinated, authorities are pressing to nudge up vaccination rates further to increase the chances that the virus doesn’t buckle healthcare systems and shutter economies.

Doctors in some parts of the continent are anxious that even a modest pickup in Covid-19 will squeeze hospitals already straining to meet treatment backlogs and ease the burden on overworked staff. Many expect a winter revival of influenza and other seasonal illnesses now that, in some places, they are no longer being kept at bay by the same social-distancing and masking measures deployed to fight the coronavirus.

Some expect this winter to be especially tough as a result of that mix of pressures, a last big hurdle before vaccines bring a durable respite from the pandemic.

“I think maybe next winter will be the real barometer for how it’s going to be long-term,” said Tom Wingfield, an infectious diseases physician in Liverpool, England. “This winter I think will be a little more severe.”

Europe has pushed ahead of the U.S. in vaccinating its citizens and has experienced a summer of relatively subdued Covid-19 caseloads, hospitalizations and deaths, despite the spread of the Delta variant.

Deaths from Covid-19 in the European Union averaged around 525 over the seven days through Tuesday and around 140 in the U.K. In January, daily deaths peaked at 3,500 in the EU and around 1,200 in the U.K., according to national data compiled by the University of Oxford’s Our World in Data project.

Adjusted for population, EU deaths equate to around 12 per 100,000 a day, and U.K. deaths to 21 per 100,000. That compares with 61 per 100,000 currently in the U.S.

The difference reflects wider vaccine coverage, especially of older and high-risk groups. The 27 countries of the European Union have fully vaccinated 61% of the bloc’s 448 million population, compared with 55% in the U.S., according to data from the U.S. Centers for Disease Control and Prevention and its EU counterpart. Big EU nations picked up the vaccination pace after a slow start earlier this year. France has fully vaccinated 67% of its population, Germany 63%, and Italy 66%. The U.K., which left the EU in 2020, has fully vaccinated 66% of its residents.

Some EU countries have some of the highest vaccination rates in the world. Portugal has fully vaccinated 82% of its population and both Denmark and Spain have inoculated more than three-quarters of their residents.

Boosting vaccination rates is a priority for many European governments to ensure maximum protection as winter draws nearer. Many are also beginning to roll out booster shots to the elderly and adults vulnerable to Covid-19 because of chronic illness or other medical reasons, to overcome the risk of waning immunity from the shots.

Governments are using both persuasion and coercion to drive vaccination rates higher. In France, where vaccination or a negative test for Covid-19 have been required to enter bars, restaurants and malls since August, authorities last week suspended around 3,000 workers in the healthcare sector for failing to comply with mandatory vaccination. Health Minister Olivier Véran said the suspensions represented only 0.1% of the 2.7 million jobs in the sector and many of those suspended were now agreeing to get a shot.

In Italy from Oct. 15, all workers in the public and private sectors will be required to have a so-called green pass, which attests that somebody has been vaccinated, recently recovered or tested negative for the virus. The requirement extends also to those who are self-employed. Greece has already implemented a similar Covid work policy.

The government in Germany launched a mid-September vaccination “action week” to boost uptake. Officials have made vaccination available in around 1,500 public places nationwide—in places giving away free kebabs and Ferris-wheel rides and administering shots at zoos and sports venues—in an effort to immunize as many people as possible before the colder months.

Germany’s tough privacy laws mean authorities don’t have as much leeway as the U.S. and other European governments to mandate vaccination in the workplace. German states restrict entry to some venues to those with proof of vaccination, recovery from Covid-19 or a recent negative test. Some are experimenting with more restrictive measures, like doing away with the negative test option in some settings and stopping compensation for unvaccinated workers who have to quarantine after testing positive or traveling to high-risk areas.

Aside from vaccination, European governments differ in the extent to which they have stuck with mask-wearing, social distancing and other low-intensity restrictions. The U.K. and Denmark have gone furthest in relaxing restrictions, while others, such as Italy, Spain and Portugal, have kept measures such as limits on indoor gatherings and masks in enclosed spaces such as public transport in place.

Italy and France have dangled the possibility of loosening antivirus provisions if progress against Covid-19 continues. A French government spokesman said Sunday that it could stop requiring health passes in areas where the virus is barely circulating.

“I’m reasonably optimistic things will go better in Italy and maybe in almost all of Europe this winter compared with last,” said Dario Manfellotto, the head of internal medicine at Rome’s Fatebenefratelli hospital and the chairman of Fadoi, Italy’s national association of internists.

In the U.K., Prime Minister Boris Johnson has detailed a so-called Plan B to reimpose mask mandates and work-from-home orders if a combination of Covid-19 cases and other pressures risks overwhelming the state-run health service. The government has pledged an extra £5.4 billion, equivalent to around $7.4 billion, for hospitals over the next six months to help them cope.

Public-health experts say a wider challenge for Europe is that the virus won’t be truly in retreat until it is beaten back world-wide. New variants may pop up that are resistant to the immunity built up so far.

“Already the world is divided into immunized and nonimmunized. This winter it will be even more stark,” said Flemming Konradsen, professor of global environmental health at the University of Copenhagen.