People worried that cases would spike when England ended restrictions on July 19. The opposite happened.

• Almost all COVID-19 restrictions were lifted in England on July 19. A large drop in cases followed.
• Experts suggest that warm weather, school closings, and the end of Euro 2020 might explain the drop.
• But as students return to school and cold weather hits, cases may rise again, experts say.

When UK Prime Minister Boris Johnson announced that he would be lifting almost all remaining coronavirus restrictions in England on July 19 even as COVID-19 cases continued to rise in the country, he drew some fierce criticism.

"We must reconcile ourselves, sadly, to more deaths from COVID," Johnson said. The other nations of the UK - which have their own public-health powers - took pointedly slower schedules.

But at about the same time as England's unlocking, the number of new infections reported each day in the UK started to plummet. It was the opposite of what many experts expected.

"This is a remarkably rapid decline and one that few anticipated," said Martin McKee, a professor of European public health at the London School of Hygiene and Tropical Medicine, told Bloomberg.

"Overall, it is a bit of a mystery."

Soccer may have played a role in the spike

Though it is "really difficult" to know what drove the sharp rise in cases, "there does seem to have been a spike associated with the Euros," McKee told Insider on Tuesday.

England, Scotland, and Wales were all represented in the postponed 2020 European soccer championships, and the England team made it all the way to the final match. From June 11 to July 11, large crowds of supporters celebrated their teams in stadiums and pubs, a breeding ground for infections, prompting World Health Organization experts to express concern.

The end of the tournament allowed cases to fall, McKee said.

Summer, warm weather, and individual nerves about unlocking probably contributed to the rapid fall

The effects of lifting the restrictions may also not have been felt yet. A heat wave hit the UK in July, which most likely caused some people to drink and dine outdoors rather than indoors, McKee said.

People also seem uncertain about going back to pre-pandemic habits.

In a survey of 3,784 adults from the UK's Office for National Statistics, 60% said they would continue to avoid crowded places.

School closings for the summer holidays also probably helped bring down cases, McKee said. "It seems likely that schools played a much greater role in transmission than some people were willing to accept," he said.

Has herd immunity been reached? Unlikely

Some experts posit that the UK may have reached herd immunity, the point at which enough people have immunity against infection that the virus is incapable of spreading.

Almost 60% of the UK's population has been fully vaccinated, and many have developed immunity from being infected.

"You can run some very simple models to see if the case numbers that we saw earlier this month are consistent with effective herd immunity," Mark Woolhouse, a professor of infectious-disease epidemiology at Edinburgh University, told The Observer.

"There are some big caveats, but the bottom line is that those figures are consistent with the impact of herd immunity," he said.

McKee thinks this is "unlikely." Cases are still rising in Israel, where vaccination levels are even higher, he noted.

And vaccinated people can be reinfected, though at a substantially reduced risk of severe disease, per McKee.

Some experts aren't yet convinced

Tim Spector, a professor of genetic epidemiology at King's College London, expressed suspicion about how representative the recorded figures were.

"The drop is much faster than we've ever seen in previous waves, even after full national lockdowns, leaving the accuracy of the official tally in doubt," he told the BBC.

Spector leads the Zoe COVID study, which tracks symptoms for 4 million people globally. Though there has been a drop in cases among the study participants in the UK, it is not as sharp as in the government stats.

Experts worry that problems will surface again in the fall, as children come back to school and the UK winter weather forces people indoors again.

But all in all, it is difficult to know what the future holds for the UK.

"At this point, I think it's really hard to understand what has happened and what is going to happen in the long term," John Edmunds, a professor of epidemiology at the London School of Hygiene & Tropical Medicine, told The Observer.