For most of 2020, avoiding the novel coronavirus was at the heart of almost every piece of public-health advice. Then, vaccinations largely gave Americans their lives back. Breakthrough infections were remarkably rare in the early months of mass vaccination. Only about 10,000 people—or 0.01% of the 101 million U.S. adults who had been fully vaccinated—reported one by the end of April 2021, illustrating that post-vaccine infections were possible, but unlikely.

That changed when the more contagious Delta variant began spreading over the summer and sickening more people who’d had their shots. Now—though vaccinated people remain far more protected than those without their shots—the highly transmissible Omicron variant may force a complete rethinking of br…