Here are the basic numbers: The U.S. has fully vaccinated 59 percent of the country and recorded enough cases to account for 14 percent of the population. (Though, given limited testing, those case numbers almost certainly underestimate true infections.) What we don’t know is how to put these two numbers together, says Elizabeth Halloran, an epidemiologist at the Fred Hutchinson Cancer Research Center. What percentage of Americans have immunity against the coronavirus—from vaccines or infection or both?

This is the key number that will determine the strength of our immunity wall this winter, but it’s impossible to pin down with the data we have. This uncertainty matters because even a small percentage difference in overall immunity translates to a large number of susceptible people. For example, an additional 5 percent of Americans without immunity is 16.5 million people, and 16.5 million additional infections could mean hundreds of thousands more hospitalizations. Because unvaccinated people tend to cluster geographically and because many hospital intensive-care units run close to capacity even in non-pandemic times, it doesn’t take very many sick patients to overwhelm a local health-care system.

What’s happening in Europe, says Ali Ellebedy, an immunologist at Washington University in St. Louis, is also a “red sign.” Several countries in Western Europe, which are more highly vaccinated than the U.S., are already seeing spikes heading into winter. Cases in Germany, which has vaccinated nearly 70 percent of its population, have increased sharply, overwhelming hospitals and spurring renewed restrictions on the unvaccinated. The U.S. does have a bit more immunity from previous infections than Germany because it’s had bigger past COVID waves, but it still has plenty of susceptible people.