Two concepts about viral spread help explain why timing and chance make such a difference. First, the coronavirus spreads exponentially, which means that even a slight delay in mitigation efforts can lead to dramatically different outcomes. Second, the virus’s spread is what epidemiologists call “overdispersed,” which means that the majority of patients do not infect anyone else but a small handful might infect dozens of people. In other words, most sparks of infection do not catch fire. But occasionally a single infection might cause an early super-spreader event, which ends up seeding a major outbreak. “Looking from state to state, it can be like, ‘Well, why is this state doing well versus that state?’ Sometimes it’s just luck,” says Adam Lauring, a virologist at the University of Michigan. In predicting how variants will behave, much of the world has looked to the U.K., where an excellent and comprehensive genomic-surveillance program has tracked the rise of Alpha and now Delta. Alpha made up 98 percent of all COVID-19 cases in the U.K. at that variant’s peak in March; Delta has since taken over, accounting for almost all new cases. It’s too early to say whether the U.S. will follow the same trajectory. Alpha was responsible for anywhere from 38 to 86 percent of all new U.S. cases last month, depending on the state. Nathan Grubaugh, an epidemiologist at Yale, says this fact suggests the limits of comparing the two countries. “The U.S. is far more heterogeneous than the U.K.,” he told me, with more diversity in viruses and bigger geographic differences in vaccine uptake. When it comes to Delta, he said, “that means some places are going to be impacted harder.” And most likely, those places are going to be the ones where fewer people have been vaccinated.