Health officials warn of potential measles exposure at O'Hare

Chicago health officials are warning the public about potential measles exposure at O'Hare International Airport after confirming a case in a traveler who arrived at Terminal 5 on June 17. The exposure window runs from 5:50 a.m. to 9:30 a.m. that day, and anyone who was in Terminal 5 during those hours should check their vaccination status immediately.

This is the second potential measles exposure at O'Hare's international terminal this year, and it comes amid a nationwide surge in cases that has public health officials deeply concerned. As of June 18, there have been 2,104 confirmed measles cases in the U.S. this year. The largest outbreaks are in Utah and South Carolina. Last year saw 2,288 cases — the highest number in decades, according to the International Vaccine Access Center at the Johns Hopkins Bloomberg School of Public Health.

The trajectory is clear, and it's accelerating. After measles was declared eliminated in the United States in 2000 — meaning there was no continuous transmission for at least 12 months — the disease has made a sustained comeback driven by declining vaccination rates and increased international travel. The current case count of 2,104 with half the year still to go suggests 2026 could surpass 2025's record.

Measles is one of the most contagious diseases known to medicine. The virus can remain airborne in a room for up to two hours after an infected person leaves. If one person with measles is in a room of 10 unvaccinated people, nine of them will become infected. The basic reproduction number — the average number of people each infected person transmits to in a susceptible population — is between 12 and 18, making measles far more contagious than COVID-19 (R0 of 2-3), flu (R0 of 1-3), or even the Delta variant at its peak.

The Chicago Department of Public Health is working with the Illinois Department of Public Health and the U.S. Centers for Disease Control and Prevention to identify and notify individuals who may have been exposed. If you were in Terminal 5 at O'Hare on the morning of June 17, here's what you need to know.

First, check your vaccination status. Two doses of the measles, mumps, and rubella (MMR) vaccine are considered 97% protective against measles. One dose is 93% protective. If you were born before 1957, you are generally considered immune due to likely prior infection. If you received two MMR doses after your first birthday and at least 28 days apart, you are considered fully vaccinated. If you're not sure of your status, contact a healthcare provider — they can check your immunity through a simple blood test.

Second, watch for symptoms. Measles has an incubation period of 7 to 21 days after exposure, which means anyone exposed on June 17 could develop symptoms as late as July 8. Initial symptoms include high fever, cough, runny nose, and red or watery eyes — easily confused with a cold or flu in the early days. The characteristic rash typically appears 2-4 days after the initial symptoms, starting on the face and spreading downward.

Third, if you develop symptoms, call your healthcare provider before going to their office. Measles is so contagious that showing up unannounced at a clinic or emergency room could expose other patients. Providers need to take precautions to prevent transmission, and they can't do that if you walk in without warning.

The broader context is important. The U.S. measles surge is not happening in a vacuum. It reflects a global increase in measles cases driven by disrupted vaccination programs during the COVID-19 pandemic, ongoing conflict-related healthcare disruptions in several regions, and a persistent anti-vaccine movement that has chipped away at community immunity thresholds. For measles, the herd immunity threshold is approximately 95% — meaning that if less than 95% of a community is vaccinated, the virus can spread. Many U.S. communities are now below that threshold, particularly in areas with high rates of non-medical vaccine exemptions.

Illinois had recorded four measles cases between January and April of this year before this airport exposure. The fact that this traveler likely caught measles outside the U.S. and brought it to one of the country's busiest international airports underscores the reality that in a world of global travel, no community is protected unless vaccination rates are high enough to maintain herd immunity.

What This Means For You: If you traveled through O'Hare Terminal 5 on the morning of June 17, check your vaccination records today. If you're not sure you've had two MMR doses, contact a healthcare provider. The vaccine is effective if administered within 72 hours of exposure for people who haven't been vaccinated, but that window is closing. If you're outside the 72-hour window, immune globulin can be given within six days of exposure for high-risk individuals including infants under 12 months, pregnant women without immunity, and immunocompromised people. Beyond this specific exposure, the broader lesson is clear: measles isn't a historical curiosity. It's an active threat in 2026, and the single most effective thing you can do to protect yourself and your community is ensure your MMR vaccination status is up to date.