Experts say the current measles divide stems from pockets of under-immunization and the airborne nature of the virus, which enables quick spread in susceptible communities, while areas with about 95% vaccination coverage help prevent broader outbreaks.

The contrast between Pennsylvania’s surge and the relatively contained counts in New York City and Washington, D.C. illustrates the localized nature of outbreaks and the protective effect of high community immunity around 95%.

Maryland reported 40 confirmed measles cases by mid-September 2026, with clusters concentrated in St. Mary’s, Cecil, and Carroll counties.

New York City and Washington, D.C., have kept case counts comparatively low despite regional spread, with NYC at six cases and D.C. at one case as of mid-September 2026.

About 95% of confirmed measles cases nationally involve individuals who are unvaccinated or have unknown vaccination status, and the majority of cases are linked to dozens of new outbreaks in 2026.

Pennsylvania has emerged as a major hotspot with at least 767 confirmed cases and four deaths across 38 counties by late September 2026, including potential exposures at CHOP and Philadelphia International Airport.

Public health responses include DC Health issuing a Health Awareness Notice to urge aggressive screening and isolation, while NYC Health relies on routine monitoring and localized alerts to prevent wider spread.

This year marks the United States’ most severe measles year in over three decades, with 3,471 confirmed cases across 47 jurisdictions as of mid-September 2026, rising from 2,289 in all of 2025.

Measles is highly contagious and airborne, remaining infectious in air or on surfaces for up to two hours, threatening infants under 12 months, pregnant women, and immunocompromised individuals.

Outbreaks typically begin when an infected international traveler enters communities with lower routine MMR vaccination coverage, underscoring the importance of maintaining high community immunity.