The majority of coronavirus infections in a major outbreak earlier this month in Massachusetts were among vaccinated people, according to an analysis published by the Centers for Disease Control and Prevention. Three-quarters of the people infected in Provincetown were vaccinated, and the infection cluster, caused by the more contagious delta variant, was a major factor in the CDC changing its masking guidance this week. The breakthrough infections caused few hospitalizations, indicating that the vaccines provide excellent protection against severe illness. The agency now recommends that vaccinated individuals wear masks alongside the unvaccinated in areas with high COVID-19 transmission.