The ongoing surge in U.S. measles cases stems primarily from sustained transmission in under-vaccinated communities, with CDC data through mid-May 2026 confirming 1,893 cases nationwide—already exceeding 2024 totals and tracking toward last year’s full-year count of 2,288. Nearly all recent infections are outbreak-associated across 40 jurisdictions, with 92% occurring among unvaccinated individuals or those with unknown status and about 6% requiring hospitalization. Official surveillance reports highlight continued weekly additions from domestic spread rather than isolated travel imports. CDC case counts update Thursdays, so final tallies through May 31 will incorporate any late-reported infections meeting the confirmed laboratory or epidemiological criteria.