More alarming are reports of a gaping, and growing, partisan divide. A poll this month from NPR, PBS NewsHour, and Marist found that 87 percent of registered Democrats had either received a vaccine or planned to get one, compared with just 56 percent of registered Republicans. Similar spreads—of 25 to 30 points—have shown up in several other surveys too, and the size of the partisan gap appears to have increased by half since the fall. Donald Trump, who has said little on this topic since leaving office, finally appealed to his supporters on Fox News last week: It’s a great and safe vaccine, he said. “I would recommend it, and I would recommend it to a lot of people that don’t want to get it—and a lot of those people voted for me, frankly.”

Yes, let’s be frank: If vaccine acceptance tops out where it is right now, at less than two-thirds of American adults, then the pathway out of this pandemic could stretch and twist into the future. The virus will remain among us, if defanged for many, and harmful outbreaks could emerge as antibody levels fade. If patterns of refusal continue to develop along partisan lines, our outlook will be even worse. Because Republicans and Democrats tend to cluster in different places, even down to the level of neighborhood, a large partisan gap in vaccine uptake would likely lead to hot spots of infection. (When people who refuse a vaccine live near one another, the risk goes up.) But a better outcome also seems well within our reach. Although no one knows how much immunity would be enough to make the disease go away, Anthony Fauci has lately said, “If you want our society to get back to normal, you have to get about 70 to 85 percent of the population vaccinated.”

So the coming months will be dispositive. An undecided segment of Americans—vaccine swing voters, who now make up roughly 20 percent of all adults—have enormous power to determine how this goes. Which way will they break?