Measles is one of the most transmissible viral diseases, spread via direct contact or airborne transmission. Vaccination with two doses of the measles-mumps-rubella (MMR) vaccine is 97% effective for preventing infections.¹ Although infrequent, measles transmission on aircraft is well documented.^5,6,7,8 Over the past two decades the highest number of secondary measles cases identified from exposure to a single index case-patient on a single flight was five travelers on a flight from Malaysia to the U.S., all of whom were unvaccinated children.⁹ Secondary infections due to exposures in airports have also been documented.^10,11,12 Secondary infections occurring on aircraft have been documented in both vaccinated and unvaccinated people.⁶ Despite a highly effective vaccine, infections can occur in vaccinated persons after exposure, particularly if the vaccinated person was exposed to a high level of infectious virus. Vaccinated people who contract measles generally have milder symptoms and fewer complications, and rarely transmit measles to other people.^2,3,4

The U.S. Centers for Disease Control and Prevention (CDC) works with airlines and U.S. health departments to initiate aircraft contact investigations (CIs) when travelers who flew while infectious with measles on domestic or U.S. inbound international flights are identified and CDC is notified within 21 days of the flight.¹³ The infectious period is defined as four days prior to onset of rash to four days after rash onset.² The defined contact zone for measles currently used for these aircraft CIs includes passengers sitting within two rows of the ill traveler, any infant in arms seated anywhere on the plane, and any cabin crew who served the index case-patient. For aircraft with 50 or fewer passenger-capacity, or flights with unassigned seating, all passengers and crew are considered exposed. To identify exposed travelers, CDC requests a flight manifest from the airline(s) and then shares domestic travelers’ contact information with health departments of jurisdiction where the travelers are currently residing via Epi-X, a secure data exchange platform. Exposed foreign travelers are also identified, and those travelers’ countries of origin are notified and investigate using their country-specific contact investigation protocols. U.S. health departments receiving traveler contact information communicate with exposed travelers to notify them of the exposure, review history of vaccination or prior measles illness, recommend and help coordinate post-exposure prophylaxis (PEP) for eligible travelers, and provide information on measles symptoms and guidance to contact the health department if symptoms develop. The exact means of communicating with and interviewing exposed travelers varies by jurisdiction, but CDC requests health departments collect a minimum standardized set of variables for cases in which travel is reported. The window for effective PEP is three days after exposure for contacts eligible to receive the MMR vaccine, or six days for those considered at high risk for severe disease or complications (pregnant women, infants, and immunocompromised people) and for whom immunoglobulin is recommended.¹⁴ Health departments may follow up with susceptible exposed travelers during their incubation period to gather additional information, including identification of any secondary infections that develop. Data are reported back to CDC on a voluntary basis and maintained in CDC’s Port Health Activity Reporting System (PHARS), an electronic system used to store information on activities conducted at CDC’s 20 port health stations.

In May 2025, CDC was notified of a traveler who flew while infectious with measles on an international flight from Europe to Colorado, transited through the Denver International Airport before and after an overnight hotel stay, and then flew on a domestic flight from Colorado to North Dakota, exposing travelers on both flights, in airports, and potentially at the hotel. Given the rapid identification of multiple secondary case-patients on the international flight, the standard measles airplane CI zone, comprising 50 passengers and cabin crew, was expanded to include an additional 66 passengers who were seated in the same section as the index case-patient. For the domestic flight, because the passenger capacity size was less than 50 passengers, all travelers on the flight were included in the airplane CI zone (total of 40 passengers and crew).

This report describes in detail the resulting CI outcomes, including laboratory testing results for secondary and tertiary case-patients, and an environmental assessment.

Information recorded in PHARS was extracted to describe the index case-patient, contacts, and travel details. Variables extracted from PHARS and included in the descriptive analysis were age, sex, vaccination status, exposure setting, time from exposure to contact notification, recommendations for PEP, and time to rash onset.

Environmental assessment included a calculation of wait times between boarding and departure based on flight schedules. FlightAware (https://www.flightaware.com/live/flight/) was used to determine scheduled boarding and flight departure times, arrival and departure gates, taxi time, and time from landing to gate. Time for boarding and deplaning was estimated from aircraft manuals for the affected aircraft. ^19,20 Total ground time was calculated by adding boarding time, taxi time, time from landing to gate, and deplaning time. Visual inspection of affected gates in the Denver International airport terminal, plane maps, and aircraft seating charts were used to determine proximity of contacts and secondary case-patients to the index case-patient.

Data were extracted from PHARS using Microsoft SQL Server Management Studio. Descriptive analyses to track case counts and calculate attack rate, mean, median, range, and percentages were completed using Microsoft SQL Server Management Studio and Microsoft Excel.

Due to the large number of secondary case-patients identified, CDC performed additional laboratory testing on specimens from identified cases which included N450 genotyping,¹⁵ whole genome sequencing,¹⁶ IgG avidity,¹⁷ and plaque reduction neutralization (PRN).¹⁸

This activity was reviewed by CDC, deemed not research, and was conducted consistent with applicable federal law and CDC policy (see e.g., 45 C.F.R. part 46, 21 C.F.R. part 56; 42 U.S.C. §241(d); 5 U.S.C. §552a; 44).

This case of an unvaccinated traveler who flew commercially while infectious and symptomatic with measles, transmitting the virus to 15 others during travel with subsequent community transmission, and exposing travelers from at least 15 different U.S. jurisdictions, highlights the continued importance of monitoring and tracking measles exposures during travel to quickly identify and mitigate spread. All but one of the secondary case-patients exposed during a flight were identified using the contact zone currently defined in CDC protocols for measles aircraft CIs. Given how contagious the measles virus is, missing a single secondary case of disease may not be acceptable. Expanding the contact zone in flights with multiple primary cases or multiple secondary cases may increase identification of additional secondary and tertiary cases. Approaches such as those currently used by the United Kingdom (notification of all passengers and crew on an aircraft in which an infectious traveler was present) or Canada (public notification when any traveler is identified who was infectious during a flight), could also be considered to broaden notifications beyond the aircraft contact zone currently used in the U.S.^24,25

The substantial number of secondary cases of disease among travelers and others exposed in the airport in this case emphasizes the need for good ventilation and airflow through the terminals, especially in older, smaller or more confined areas.²⁶ Due to high air exchange rates (typically 20-30 times an hour) when an aircraft’s ventilation system is on, transmission of airborne pathogens is less likely on commercial passenger aircraft than in buildings (e.g., airports).²⁷ However, during boarding and deplaning, when there is closer contact between travelers both in the jet bridge and in aircraft aisles, the aircraft ventilation system air exchange is typically turned off to conserve fuel, and the aircraft relies on ground ventilation that is much less efficient. A high degree of infectiousness of the index case-patient, a reported cough, and time spent in shared airspaces likely contributed to large number of secondary cases of disease among those exposed. Infectiousness likely peaks during late prodrome, prior to the development of neutralizing antibodies (heralded by the maculopapular rash), which was the time of the international and domestic flights for the index case-patient.²⁸ Delay in measles diagnosis in an index case-patient until several days after the flight is not an uncommon occurrence.⁵ PEP is only effective if administered within three to six days after exposure. The delay in diagnosis and subsequent delay in CDC notification for this index case-patient meant that notification of exposed passengers or crew occurred beyond the window for effective PEP. Test results indicated a secondary vaccine failure in three of the four secondary case-patients with documented MMR vaccines, which is known to occur in a small percentage of the vaccinated population.²⁹ Generally, measles vaccination is considered to provide robust protection against all wild-type measles viruses, and limited mutations detected by WGS suggest that viral mutations do not account for the large number of secondary cases of disease or high proportion of cases among previously vaccinated people. ³⁰ The low number of detected tertiary cases may be due to lower infectiousness of people with secondary vaccine failure, implementation of public health measures to prevent spread to unvaccinated close contacts (e.g., PEP, quarantine), or pre-existing immunity among tertiary contacts, and highlights the importance of vaccination.³¹

All secondary case-patients and the two tertiary case-patients had positive RT-PCR test results, and the majority of secondary case-patients’ positive laboratory results were identified through the use of multiple testing methods, including serology and both urine and NP/throat swab RT-PCR, which are all identified by CDC as preferred testing specimens.³² Measles virus can be detectable in the urine even when not detectable by NP swab, particularly when sample collection is several days after rash onset or in mild cases.^33,34 WGS results indicated that viral sequences from the index case-patient and secondary case-patients were related, and consistent with current circulating virus strains.

This investigation entailed a large expenditure of resources at both the federal and state level. Ongoing economic evaluation of aircraft contact investigations in the U.S. is currently underway. This assessment will help determine the most cost-effective strategies for managing measles exposures on commercial aircraft in the future. This is important particularly in the current situation in which measles cases are rising in the U.S. and globally, and the U.S. faces impending loss of measles elimination status.

This report has several limitations. Information identifying aircraft passengers outside defined contact zones is not routinely provided by airlines, and aircraft passengers from international jurisdictions are investigated by their home country and not by the CDC, thus additional cases among aircraft passengers may have been missed, including among passengers seated outside the defined contact zones on the international flight, or who had departed the United States at the time the CI was conducted. CDC’s measles case database was cross-referenced to identify potential missed flight-associated cases; additionally, as part of routine aircraft CI procedures, CDC notified public health authorities in destination countries for passengers who were non-U.S. residents. No additional cases were identified through these mechanisms. We were unable to identify all travelers present at the Denver International Airport during the exposure window, thus unable to gather vaccination status on those unidentified travelers. Additionally, the index case-patient traversed two additional airports and stayed in a hotel for which no exposed traveler information was available. We were unable to determine vaccination status of all identified exposed travelers who did not develop measles disease, thereby limiting our ability to determine whether secondary infectiousness was more common among unvaccinated contacts. Vaccination status of many secondary case-patients was identified through verbal recall, which can be an unreliable or incorrect source of immunization information. Improved implementation of immunization information systems over time could potentially help to address this issue in the future. While three unvaccinated and two self-reported vaccinated secondary case-patients were reported hospitalized, other indicators of severity of symptoms were not available for the majority of secondary case-patients. The missing information related to vaccination makes it difficult to determine whether infection severity was worse in unvaccinated secondary case-patients as would be expected.³⁵