But Fauci, as well as most of the other experts I talked with, cautioned against overinterpreting the results of a single study, especially one documenting only a snapshot in time. Even taken at face value, the “better” defenses offered by post-infection immunity come at a massive potential cost, said Goel, of the University of Pennsylvania. Cells and molecules are scrambling to learn the traits and weaknesses of a foreign invader while their home is being attacked; any infection bears some risk of hospitalization, long-term disability, or death. The virus can also interfere with the immune response, muffling antiviral defenses, severing the ties among disparate branches of immune cells, and, in some cases, even duping the body into attacking its own tissues. And unlike the vaccines, infections are, well, infectious, turning each afflicted person into “a public-health threat,” Nahid Bhadelia, the founding director of Boston University’s Center for Emerging Infectious Diseases Policy and Research, told me.

Those who surface from these encounters seemingly unscathed might not have much immunity to show for it, either. Several studies have shown that a decent percentage of infected people might not produce detectable levels of antibodies, for the simple reason that “not all infections are the same,” Beatrice Hahn, a virologist at the University of Pennsylvania, told me. The immune system tends to use its own threat assessment to calibrate its memory, dismissing many brief or low-symptom encounters. That could be an especial concern for people with long COVID, many of whose initial infections were asymptomatic or mild.

At the other end of the spectrum, very severe disease can so traumatize the immune system that it fails to recollect the threat it’s fighting. Researchers have watched immune-cell training centers “completely collapse” beneath the blaze of inflammation, Eun-Hyung Lee, an immunologist at Emory University, told me. In some cases, the virus might find its prior hosts nearly as unguarded as before. “It would be dangerous to assume good immunity across all individuals in this group,” says Kimia Sobhani, who’s studying antibody responses to the virus at Cedars-Sinai Medical Center, in Los Angeles.