The United Kingdom recently reversed a surge in Covid-19 cases caused by the delta variant—and while experts don't fully understand why the British surge faded, they point to three potential explanations, David Wallace-Wells reports for New York Magazine's "Intelligencer."

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A virus that surges—and sometimes fades—unexpectedly

The surge of delta variant Covid-19 cases in the United Kingdom started in mid-May, hit its peak in mid-July, and then rapidly declined, Wallace-Wells reports.

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The precipitous fall in cases took most experts by surprise. For instance, Neil Ferguson, a prominent adviser to the British government and an epidemiologist, had recently predicted the U.K. could soon see 200,000 Covid-19 cases a day—yet now, it's seeing only 27,000 per day.

Yet such sudden turnarounds have occurred frequently over the course of the pandemic, Wallace-Wells reports, with infections surging and fading for reasons that nobody fully understands. In January, for instance, the CDC assembled 28 projections of the near-term course of coronavirus infections. Of those, just two included in their range of possibilities the course the epidemic took just two weeks later.

Because the coronavirus has behaved in such unexpected ways in the past, few experts are willing to definitively predict the delta variant's future course in the United States. Yet the U.K. experience offers glimmers of hope that case counts can be brought back down, Wallace-Wells reports.

3 reasons why experts think the UK delta variant has faded

Wallace-Wells spoke to a number of scientists to understand the U.K.'s recent turnaround. While none could point to one specific cause, most referenced three distinct factors: closures and other coronavirus countermeasures implemented by the U.K. government, vaccinations, and changes in human behavior.

"I don't think we have the definitive explanation for the recent trends in the U.K.," Lauren Ancel Meyers of the University of Texas acknowledged. "[But] you see this sort of feedback between what the virus is doing and how people are behaving—we're sort of course-correcting as we go."

She added, "You see the surges and you feel the risks personally, you hear about the hospital feeling overwhelmed, and sometimes there are policies that are guiding how you behave, as well. And people go from being more relaxed and taking greater risks to being more cautionary, and taking fewer risks."

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Adam Kucharski of the London School of Hygiene and Tropical Medicine noted that vaccination rates have been a significant factor in shaping the coronavirus's spread.

"Last year we were in a situation where, basically, there was widespread susceptibility in the population, and so if you wanted the numbers to come down, you needed widespread behavioral change," he said. "Whereas now, because we've got a lot of accumulated immunity in the U.K., particularly among older groups, that means a lot of transmission and susceptibility is really concentrated in these younger groups … What's going on in those groups, in those pockets, really drives what's going on in your epidemic, because we're probably at a point where it would be quite difficult to get self-sustaining transmission now in some of these older groups, just because the vaccine coverage is so high."

However, according to Kucharski, vaccines can't fully explain the decline in cases in the U.K.. He cited school closures and the end of the European soccer championships—which drove many young men to congregate—as additional factors contributing to the decline in cases.

The U.K.'s contact tracing app was also helpful in getting cases down, Kucharski said. For the week ending in July 21, 700,000 notifications were sent out in the app, he said. According to Kucharski, with additional precautions, "the susceptibility network is much more fragmented," with behavioral changes made by those groups of people having a larger impact than they would have earlier in the pandemic.

Another hypothesis is that the U.K. has reached a limited version of herd immunity with roughly 80% of the population having gained immunity either from a vaccine or from previously having Covid-19, Wallace-Wells reports.

A virus that still surprises experts

Either way, the rapid drop in cases is something experts didn't expect, Wallace-Wells reports.

"No one expected it to go up so quickly, and no one expected it to go down so quickly," Youyang Gu, a pandemic modeler, said. "I think the short answer is just that the pandemic is unpredictable, even for scientists." (Wallace-Wells, "Intelligencer," New York Magazine, 8/1)