When the U.K. dropped all coronavirus restrictions on “Freedom Day,” July 19, critics called the move a “dangerous and unethical experiment.” Harsher critics called it “epidemiologically stupid.”

At the time, cases in the country were still rising amid a Delta-fueled spike. Then, to nearly everyone’s surprise, COVID-19 cases started falling. This suggests that Delta hit a natural peak in the U.K. by mid-July—not because of Freedom Day, the effects of which are only just starting to show up in the data, but through some other mechanism.

Now, two weeks later, “the impact of Freedom Day is the big question,” says Paul Hunter, an epidemiologist at the University of East Anglia. “And I’m not at all sure which way it will go.” We will soon get the first glimmers of data that show how Delta behaves when all restrictions are lifted in a highly vaccinated country. This is uncharted territory. What happens in the U.K. can raise the rest of the world’s hopes—or dash them.

The optimistic scenario: Cases keep decreasing through August as the U.K. vaccinates more people, which would be “a really reassuring thing for the whole world,” says Jeffrey Barrett, the director of the COVID-19 Genomics Initiative at the Wellcome Sanger Institute. “It suggests it’s possible to actually get past Delta. We’re not going to be stuck forever.” With its high vaccination rates, especially among the elderly and most vulnerable, the U.K. is one of the best-positioned countries in the world to ride out the Delta wave without lockdowns. Hospitalizations during this Delta-driven bump remained low compared with pre-vaccination waves.

But if the opposite happens, if cases skyrocket so much that hospitalizations also rise to overwhelming levels, then even higher vaccination coverage and future restrictions might be necessary, especially in the fall. The situation will be worse in countries with lower vaccination rates.

In this way, whatever happens in the U.K. represents a best case for the U.S., a country with lower and patchier vaccine uptake despite having plenty of doses. Delta took over in the U.K. first, so its trajectory is slightly ahead of the United States’. If even the highly vaccinated U.K. cannot keep the virus under control in the coming months, that spells real trouble ahead for Americans. But if the U.K. continues on a reassuring trajectory, then the U.S. might have a shot at containing Delta, provided it quickly raises its vaccination rates among the most vulnerable. If not, even in the reassuring scenario the situation here will be much worse.

The fact that the U.K.’s Delta cases started falling earlier than expected is a pleasant surprise—even a reason for optimism. It’s unlikely the U.K. has reached herd immunity with 58 percent of its population fully vaccinated. But cases dropped nonetheless, and experts pointed to several other factors that may have played a role: schools closing for the summer, weather, and a “pingdemic” in which many exposed people were told to isolate by the National Health Service’s contact-tracing app. The most encouraging explanation, in terms of curbing Delta, is that the peak was accelerated by a onetime event: the Euro 2020 soccer tournament.

England’s team made it all the way to the Euro finals, on July 11. “There were just this series of nights where people were going to football-watching parties,” Barrett says. “Hundreds of people packed in small spaces, shouting things and hugging each other.” During that period, young men aged 20 to 34 made up a disproportionate number of cases in England compared with women of the same age, further bolstering the Euro hypothesis. And in Scotland, whose team was eliminated earlier, cases also started falling earlier too, about 10 days after its team exited the tournament in June. All of this suggests the Euros were indeed a temporary shock that allowed Delta to infect more people than it would have under ordinary circumstances; once it ended, cases started to come down. The outbreak did not continue to spiral out of control, even though the U.K. still has plenty of people susceptible to COVID-19—for example, it hasn’t vaccinated any kids.

Freedom Day meant the end of all legal restrictions in the U.K., but not everything is back to normal yet. Government officials have continued to urge people to be careful. Many office workers are working from home. Schools are closed for the summer. “We’re roughly making half the numbers of contacts than we were compared to pre-pandemic levels,” says Graham Medley, an infectious-diseases modeler at the London School of Hygiene & Tropical Medicine. “But the next big challenge will be at the beginning of September.” How the U.K. does as a highly vaccinated country will again show the world whether it’s possible to get through Delta as the weather gets colder and schools reopen.

Medley expects case numbers to be bumpy, rising and falling like a series of small hills rather than one big mountain. This is because the U.K. has a high overall vaccination rate, but it still has pockets of vulnerable people. The virus will cause an outbreak every time it finds a pocket, though the exact timing might be hard to know. “It will happen,” he says. “It’s not a question of whether it happens. It’s just that we can’t predict where and when.”

The U.S. has many pockets of low vaccination, and this very uneven coverage leaves unvaccinated Americans especially vulnerable. The more concentrated susceptible people are, the easier it is for the virus to find its next victim. For example, “it’s better to have three out of four people in each household vaccinated,” Medley says, “than it is to have three out of four households completely vaccinated.” The virus cannot spread very much in those partially vaccinated households, but it will spread through the entirely unvaccinated one. And the U.S. has too many households and communities with low vaccine uptake. Many of the counties where the virus is now surging have vaccination rates below 30 percent.

There is another difference between U.K. and U.S. vaccination campaigns, this time possibly in Americans’ favor, which is that many Brits received the slightly less effective AstraZeneca vaccine. The U.S. has not authorized or used any doses of AstraZeneca. “I confess that I have little intuition for what that is going to do,” Bill Hanage, an epidemiologist at Harvard, told me in an email. But the U.S. is fortunate to have ample supplies of highly effective mRNA vaccine, and highly vaccinated parts of the U.S. may follow the U.K.’s so-far promising Delta trajectory. On the other hand, Hanage pointed to Florida, which is breaking hospitalization records despite the availability of vaccines. The U.K. may represent a best-case scenario for Delta, but it’s clear that we can also do a lot worse.