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.