One Traveler, Ten Colorado Cases: Measles from a Flight into Denver, 2025

In May 2025, an unvaccinated traveler with measles arrived in Denver on an 11-hour international flight, spent a night in a hotel, and flew on the next day. Secondary cases of measles among flight contacts are rare. This time, nine Colorado residents caught it — on the plane or in the airport — and one of them passed it on at home. Four of the nine had had both doses of MMR vaccine. Two of those four would have been missed if the only test had been a nose swab.

Secondary cases by place of exposure and vaccination status; positions are not exact seat or terminal locations. Credit: CDC, Morbidity and Mortality Weekly Report*.*

Measles is back

Measles — fever, rash, cough, runny nose and conjunctivitis — is highly contagious, but vaccination had limited its spread in the United States since 2000. In 2025 cases rose again: 2,255 were reported that year, including three deaths, and 11% of patients were hospitalized. About 10% of patients have complications such as ear infections and diarrhea; pneumonia (5%), encephalitis (0.1%) and death (0.1%–0.3%) also occur. One dose of MMR vaccine is 93% protective and two doses 97%; vaccinated people who do get measles usually have a milder illness and are hospitalized less often.

How the outbreak unfolded

The traveler, not a Colorado resident, had been exposed during an outbreak in another state before traveling abroad, and was reportedly feverish and coughing on the way back.

Another six secondary cases and one tertiary case were reported by five other states. No Coloradans caught measles on the domestic flight.

On the plane (4 cases): the first was an unvaccinated young child sitting on a parent's lap more than two rows from the traveler; three of the four were vaccinated. In the airport (5 cases): one vaccinated, three unvaccinated, one unknown. One was the airport worker, probably exposed on arrival day; CDC concluded the other four were likely exposed the next day in a smaller, more crowded concourse where the traveler boarded the domestic flight.

The worker's rash appeared 4 days past the usual maximum incubation period of 21 days, but the first symptoms came on day 20, and no other exposure was found. The household case was at first thought unrelated: that person had traveled internationally, but not through the Denver airport on the traveler's days.

Who got sick

The nine secondary patients had a median age of 29 (range 1–55). All had rashes; two vaccinated patients had mild, atypical rashes they likened to acne or insect bites, and vaccinated patients generally had fewer and milder symptoms. Four were hospitalized for 2–5 days — three unvaccinated patients, including the child, and the patient of unknown status — with high fever, dehydration, diarrhea, loss of appetite and acute viral hepatitis. All recovered. All six specimens genotyped were D8, the most common genotype in recent U.S. outbreaks.

Urine found what nose swabs missed

Every case was confirmed by RT-PCR. Nose and throat swabs are usually preferred, and urine is not routinely tested when swabs have been taken. But urine was positive in six patients, including two vaccinated patients whose nasopharyngeal swabs were negative. In the patient identified late, urine was still positive 24 days after the rash, and an antibody test confirmed recent infection. Vaccinated people, with milder illness, may also test negative for measles IgM antibodies — so routinely collecting urine could find more cases.

Tracing 1,400 contacts

CDPHE identified about 1,400 contacts in Colorado and tried to reach all of them by phone, text or both.

• Air contacts: CDC listed 92 across both flights — lap children anywhere on the plane, passengers within two rows and crew serving the patient (all on board for planes of 50 seats or fewer). After at least three calls, 74 (80%) were assessed; 18 could not be reached. All were found too late for preventive treatment.
• Contacts of later cases: 61 were eligible for post-exposure prophylaxis. Twelve were advised to get immune globulin (10 did); 49 were advised to get MMR vaccine, and 41 did, 6 declined and 2 are unknown.
• Immunity was accepted from records of a dose of MMR, lab evidence, confirmed past disease or birth before 1957; for people without close contact, a verbal report of vaccination, U.S. military service or entry on an immigrant visa or green card was enough to avoid quarantine. People without proof were monitored for 21 days.

No further cases were found. Health alerts, press releases and a public list of exposure sites went out, and public health agencies worked with the hospitals that had admitted three of the four hospitalized patients to alert staff and other patients who might have been exposed. A survey of staff and patients from the late-identified patient's hospital stay found no one else with measles.

The lesson

Measles has a secondary attack rate of 90% or more. An infectious traveler, a long flight and a crowded concourse probably all helped it spread here, even to some vaccinated people. Everyone eligible, especially travelers, should have two doses of MMR vaccine, and testing urine could help catch mild cases in vaccinated people. The report covers only Colorado residents, and limited resources meant not every exposed person could be reached.

Sources

Based on Amanda R. Metz, Meghan Barnes, Kevin Andresen, et al., "Measles Outbreak Associated with an Infectious Traveler — Colorado, May–June 2025," Morbidity and Mortality Weekly Report, volume 75, Centers for Disease Control and Prevention; a work of the United States government in the public domain.