UK coronavirus cases have been dropping dramatically over the past week. Vaccines aren't the only reason.

• The UK reported 25,000 new COVID-19 cases 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.

The number of coronavirus cases reported each day in the UK is falling almost quickly as it rose earlier this summer.

During the first two weeks of July, average daily cases reported there jumped 80%, with the single-day total 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 past week, down to just 25,000 reported Monday, as shown in the chart below.

UK COVID-19 cases reported in the past 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, the former commissioner of the US Food and Drug Administration, told CNBC on Monday. "So those people aren't presenting for testing."

But a more likely 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 tournament, which ended two weeks ago, may have temporarily driven up UK cases, with the semifinals held at London's Wembley Stadium on July 6-7. The tournament included the English, Scottish, and Welsh teams, with England advancing to the July 11 final, which was also held at Wembley. 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 in England 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, given 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. A UK study suggested that two doses of the Pfizer-BioNTech or Oxford University-AstraZeneca vaccines were 88% and 67% effective, respectively, at preventing symptomatic COVID-19 from the Delta coronavirus 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."