An unvaccinated adult who traveled from Europe to the United States while infectious with measles last year spread the virus to 17 other people, researchers from the US Centers for Disease Control and Prevention (CDC) write in The Journal of Infectious Diseases.

For the report, published late last week, the researchers analyzed case samples and data from the CDC’s Port Health Activity Reporting System and health department investigations to describe subsequent contact-tracing efforts, environmental assessments, and laboratory testing. The team also reviewed flight records to pinpoint relative locations of the index and secondary case-patients at the arrival airport.

Index patient traveled despite symptoms

In May 2025, the CDC was notified of an adult who flew commercially from Europe to Denver International Airport in Colorado, stayed overnight in a hotel, and then boarded another flight to North Dakota. The case-patient had a fever, persistent cough, cold-like symptoms, and conjunctivitis (“pink eye”) during travel, and his rash occurred one day after his domestic flight.

Travelers with fever and other overt signs of transmissible illness, such as coughing or malaise, should be strongly encouraged to delay travel while symptomatic.

Contact investigation identified 135 domestic travelers exposed to the index patient. Fifteen of them (13 adults, two children) were infected during the international (five) and domestic (three) flights and at the airport (seven). The virus then spread to two other people outside of their households, for a total of 17 infections. 

Ten of the 15 secondary case-patients had documented or self-reported receipt of at least one dose of a measles vaccine, while five were unvaccinated. Five case-patients (three unvaccinated, two who self-reported vaccination) were hospitalized. 

Of the two tertiary case-patients, one was confirmed to have received two vaccine doses and was exposed to a secondary case-patient who self-reported vaccination, while the other was unvaccinated and exposed to an unvaccinated secondary case-patient.

“Vaccination is recommended prior to international travel for all travelers aged 6 months or older,” the study authors wrote. “Travelers with fever and other overt signs of transmissible illness, such as coughing or malaise, should be strongly encouraged to delay travel while symptomatic.”

CDC and state researchers reported a similar travel-related outbreak in late January.

Health authorities in Spain confirmed to the European Centre for Disease Prevention and Control (ECDC) a human case of variant H1N1 swine influenza (H1N1v). This marks the first case of H1N1v flu in Spain since 2024; since 2009, only four human cases of swine flu have been documented in Spain. No human-to-human transmission of H1N1v has ever been documented in Spain. 

The case was detected in Catalonia in February, the same region where the case in 2024 was detected. The patient had no flu symptoms.

 The likelihood of further transmission in human[s] of variant A(H1N1) linked to this event is assessed to be very low

“The exposure to infected pigs represents the most common source of infection in humans,” according to an ECDC report. “Based on the information currently available, the likelihood of further transmission in human[s] of variant A(H1N1) linked to this event is assessed to be very low.”

No known contact with pigs 

The person had no known contact with pigs or environmental exposures. Spanish authorities have warned the World Health Organization that the person may have contracted the virus from a human, according to Reuters. 

The patient remains asymptomatic, and all close contacts have tested negative for the virus.

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