**Ongoing measles outbreaks fueled by vaccination gaps continue to drive rapid case growth in the United States.** As of September 24, 2026, the CDC has confirmed 3,659 measles cases for the year—the highest annual total since 1991—with 95% linked to outbreaks and 95% occurring in individuals who were unvaccinated or had unknown status. Pennsylvania remains the epicenter, reporting over 800 cases, while major clusters persist in states including South Carolina and Utah. Weekly additions have recently exceeded 170 cases, reflecting sustained community transmission of the highly contagious virus (basic reproduction number typically 12–18). CDC surveillance updates, including weekly case counts and any revisions from state health departments, will shape near-term market movement through the October 31 resolution window. Historical patterns show that without sharp improvements in MMR coverage, incidence can accelerate in under-vaccinated populations.