A second possibility is that the virus will partially evade our vaccine-generated immune defenses while paying a price, becoming less infectious or lethal. In order for the coronavirus to hide from our antibodies, it has to change aspects of the key components recognized by our immune systems, including the spike protein; those changes could end up diminishing the protein’s ability to bind to the receptors it needs to infect cells. We can consider, for example, the Beta and Gamma variants, which exhibit some level of immune evasion but haven’t become as infectious as Alpha or Delta. In the nineteen-nineties, H.I.V. experienced such a fate, when it hit upon a mutation known as M184V, which conferred resistance to the antiviral drug lamivudine. On the surface, this was a setback, but doctors soon learned that patients with the M184V variant had lower viral loads, suggesting that the mutation also reduced how efficiently the virus replicated inside the body. It became common for patients with H.I.V. to continue taking lamivudine even after resistance emerged, in part to select for the variant with a lower replication rate.

The third future is the most concerning: the virus could accumulate mutations that allow it to circumvent immunity without suffering a major reduction in transmissibility or lethality. This would require it to open up a new evolutionary space—a citrate moment. Even in this scenario, Burioni told me, we’re in a fortunate position: we can quickly modify our vaccines to confront new variants. At the same time, the manufacturing and distribution challenges facing those variant-specific boosters would be colossal; we’re struggling to fully vaccinate even a quarter of the world’s population with the vaccines we already have.