Cases have stayed at high levels since then, with more than 20,000 new cases recorded each day. There are almost 9,000 Britons hospitalized with Covid-19 — the highest level since March, when the United Kingdom was in the midst of a long national lockdown.

Unlike the rises and falls of previous periods of infection, the most recent wave shows the positive impact of Britain’s vaccination rollout: Far fewer Covid hospital admissions and deaths have followed the rise in cases than in previous waves. Still, health experts contend that the ongoing hospitalizations and deaths are burdening overstretched hospitals and could be reduced with basic measures.

Tim Spector, a professor at King’s College London, who has been leading a major study of Covid symptoms since the start of the pandemic, said it is hard to predict whether high levels of transmission will continue. One reason for the uncertainty, he said, is that public behavior, led by government guidance, is very different now than in previous surges. “In past waves, there’s been a general panic and reduction in mobility,” he said.

“The trouble is that the government has gone from ‘operation fear’ that we had a year ago to ‘operation complacency,’” Spector said. “It’s a massive switch.”

England has some of the loosest coronavirus protections in Europe since July 19, when it lifted all legal restrictions, including mandatory mask-wearing. In a recent survey by YouGov and Imperial College London, 21% of Britons said they rarely or never wear a face mask in public — about four times as many as in Italy and Spain.

Despite the extended surge in cases, the UK’s Scientific Advisory Group for Emergencies recently said that behaviors in Britain are closer to pre-pandemic levels than at any time since March 2020.

The current surge is being primarily driven by high levels of infection in school-age children, with more than one-third of all recent cases being reported in those younger than 15.

Unlike most of Europe, the UK was slow to approve vaccines for adolescents. The recommendation of a single dose of the Pfizer-BioNTech vaccine for 12- to 15-year-olds was not announced until mid-September — weeks after many students had returned to school from summer vacation. So far, just 21% of 12- to 15-year-olds are vaccinated in England, compared with 80% of adults.

Christina Pagel, director of the Clinical Operational Research Unit at University College London, said the lag in approval of vaccines for 12- to 15-year-olds was an enormous mistake.

“We’ve had an extra 10% or 20% of kids infected when they didn’t need to be, and we’re dealing with the hospitalizations and deaths that came from that,” Pagel said. “And with long Covid, even if that affects 5% of them, that’s a lot of kids.”

These infections have started to spread into older age groups in recent weeks, and overall in England, about 20% of beds in intensive care units are filled with Covid patients. Experts warn that the National Health Service could face intense pressure this winter.

Experts agree that Covid vaccine booster doses — an additional shot given once the protection from the initial inoculation starts to decline — are going to be essential to reduce demand on hospitals in the coming months. A recent study from Public Health England found that protection against symptomatic infection drops significantly in the five months after a second vaccine dose.

The study found the Pfizer-BioNTech vaccine’s effectiveness at preventing symptomatic infection from the delta variant dropped to 70% after five months from 90% two weeks after full immunization. Waning immunity was more severe for those who received the Oxford-AstraZeneca vaccine; five months after being administered, its effectiveness dropped to 47% from 67%.

Booster doses are currently being rolled out to everyone ages 50 and above in Britain, as well as health care workers and the clinically vulnerable, six months after their second doses. So far, 6.7 million booster doses, or enough to cover 10% of the total population, have been administered.

The UK’s high case rates had been an outlier in Western Europe in recent weeks, as infections in France, Germany and Spain all plateaued at low levels. But rising cases in Germany hint that Britain’s woes could spread across the continent this winter, with fears that waning immunity and increased indoor mixing will fuel further infection.

The British government has so far resisted calls to invoke its “Plan B” for suppressing infections over the winter. That protocol would include vaccine passes for nightclubs, mandatory face coverings and a push for those who can to work from home.

At a recent news conference, Health Secretary Sajid Javid said although the NHS is under increasing pressure, he believed the current lack of restrictions remained appropriate. “If we feel at any point it’s becoming unsustainable, then we won’t hesitate to act,” he added.

Dr Chaand Nagpaul, council chair of the British Medical Association, in a recent statement pressed for more urgent action. He said the NHS is heading into winter with a “depleted and exhausted workforce” and called on the government to enact its “Plan B” measures now to prevent the NHS from being overwhelmed.

Case rates have fallen in the UK in the past few days, although it remains to be seen whether this trend will continue once children return to school next week after a midsemester break. Pagel said she hoped the brief vacation from school would serve as a break in transmission. Still, she warned, “Even if cases do start declining, they’re not going to go down super fast. If we added in masks and home working, we could make them go down quicker, so why aren’t we doing that?”

She added: “Last winter was awful, and we shouldn’t get anywhere near that level before raising alarm bells.”