UK coronavirus cases have been dropping dramatically over the last week. Vaccines aren’t the only reason why.

• The UK’s daily coronavirus cases dropped to 25,000 on Monday after a recent peak of nearly 55,000 on July 17.
• Experts think a combination of warm weather and fewer public gatherings may have helped.
• The UK’s promising trajectory may bode well for the US, where cases are surging.
• See more stories on Insider’s business page.

The UK’s daily coronavirus cases are falling almost quickly as they rose earlier this summer.

During the first two weeks of July, average daily cases there jumped 80%, peaking at nearly 55,000 on July 17. That’s close to the levels recorded during the worst days of the UK’s winter outbreak, when vaccines weren’t yet widely available.

But cases have dropped dramatically in the last week, down to just 25,000 cases on Monday, as shown in the chart below.

UK COVID-19 cases over the last month

Experts, though surprised, have a few theories as to what happened. A recent decline in testing could be one factor: The UK administered 9% fewer tests this week than it did three weeks prior, and testing overall has declined since mid-March.

“A lot of the people who are becoming symptomatic are becoming more mildly symptomatic because they’re younger people or they’re people who have been vaccinated,” Scott Gottlieb, former commissioner of the US Food and Drug Administration, told CNBC on Monday. “So those people aren’t presenting for testing.”

But a likelier explanation, according to other experts, is a combination of warm weather – which encourages people to spend less time indoors – and fewer public gatherings.

The Euro 2020 soccer championship, which ended two weeks ago, may have temporarily driven up UK cases, since the semifinals were held at London’s Wembley Stadium on July 6 and 7, then the finals on July 11. Many schools also closed for summer holidays last week.

Additionally, the recent spike in cases may have prompted more people to self-isolate, either to avoid getting sick or because they had known exposure to someone with COVID-19.

The UK’s promising trajectory may bode well for the US

There’s no guarantee that the UK’s downward case trend will last, however, especially since most social distancing restrictions lifted on July 19. Since then, venues like restaurants, clubs, and festivals have reopened. Official case numbers generally reflect the spread of infections two weeks prior, due to the virus’ incubation period and the time it takes to get tested, get results, and see those results reported to health authorities.

“Today’s figures do not of course include any impact of last Monday’s end of restrictions,” Paul Hunter, a professor of medicine at the University of East Anglia, told the BBC. “It will not be until about next Friday before the data includes the impact of this change.”

So it’s possible that case totals will tick up again starting next week. Still, vaccines should continue to prevent fully immunized people from becoming severely ill. New research suggests that two doses of Pfizer’s or AstraZeneca’s vaccine are 88% and 67% effective, respectively, at preventing symptomatic COVID-19 from the Delta variant – the UK’s dominant strain.

The UK’s promising trajectory may even bode well for other highly vaccinated counties like the US, where cases are surging.

“If the UK is turning the corner, it’s a pretty good indication that maybe we’re further into this than we think,” Gottlieb told CNBC. “Maybe we’re two or three weeks away from starting to see our own plateau here in the United States.”