Multiple Measles Transmission Events Associated With a Single Traveler Arriving in the United States, May 2025

Travelers who fly on commercial aircraft while infectious with measles are reported to the US Centers for Disease Control and Prevention (CDC) by health departments. CDC contacts airlines to collect information on potentially exposed travelers. Traveler locating information is shared with health departments to facilitate aircraft contact investigations. In May 2025, CDC was notified of a traveler who flew from Europe to Colorado while infectious with measles, transited through Denver International Airport, then flew from Colorado to North Dakota. This report describes details of the subsequent contact investigations, environmental assessment, and laboratory testing results.

CDC laboratories conducted testing on a majority of case samples. Data from the CDC laboratory, CDC's Port Health Activity Reporting System, and health department investigations were analyzed to describe the index case, contacts, test results, and travel details. Flight records and visual inspection were used to describe relative locations of index and secondary case-patients at Denver International Airport.

The index case was in an unvaccinated adult. Aircraft contact investigations identified 135 exposed domestic travelers. Fifteen secondary cases were identified among people exposed during the international (5) and domestic (3) flights and at the airport (7). Two tertiary case-patients were also identified. Five of the secondary case-patients had at least 1 documented prior measles vaccination.

Measles transmission may occur during travel. Measles vaccination is recommended prior to international travel for all travelers aged 6 months or older. Travelers with fever and other overt signs of transmissible illness, such as coughing or malaise, should be strongly encouraged to delay travel while symptomatic.