For weeks into early August, the delta surge in the U.K. lulled some Americans into a sense of complacency. Across the Atlantic, cases soared but killed relatively few people, and in theory, the mutated virus would act similarly in the U.S. But delta exposed a key difference: The U.S. has fallen far short of the U.K. in vaccinating the oldest members of the community, who remain most at risk of hospitalization and death.

That largely explains the U.S.’s failure: About 18% of Americans 65-and-over still aren’t fully vaccinated, versus about 5% in the U.K. “That’s a huge difference,” said Jeffrey Morris, a professor of biostatistics at the University of Pennsylvania. “When you have four times as high a proportion that are unvaccinated, that’s going to cause a lot more death right there.”

The U.S. has already posted twice as many deaths per capita since early June as the U.K., even though its surge started later. In Florida, Mississippi and Louisiana, the tolls are 4-6 times as high — exacerbated by populations that tend to be older or have more pre-existing conditions.