The United States is experiencing its worst measles outbreak in over three decades, with 3,134 confirmed cases reported nationally in 2026—the highest annual total since 1991. The outbreak has been particularly severe in Pennsylvania, where cases have exceeded 1,000 across 39 counties since the outbreak began in April. Five measles-related deaths have been reported in the state, along with 198 hospitalizations.

The outbreak has become a political flashpoint following Health and Human Services Secretary Robert F. Kennedy Jr.'s keynote address at an anti-vaccine conference in mid-September, just as the outbreak accelerated. Pennsylvania Governor Josh Shapiro subsequently called for President Trump to remove Kennedy from office, citing his historical leadership of an anti-vaccine organization and the ongoing measles crisis in his state. Public health officials have attributed the outbreak's severity to declining vaccination rates across multiple states, and renewed vaccination efforts are underway nationwide as cases continue to mount.