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.