A St. Louis County resident has tested positive for measles after riding a Chicago-bound Amtrak train, marking the county’s first confirmed case of the highly contagious disease in 20 years.
The resident, who was fully vaccinated, boarded Train 318 on Aug. 27. Another passenger, later confirmed to have measles, had boarded in Sedalia, Missouri, according to the St. Louis County Department of Public Health, which announced the case last week.
County health officials are working with state and local agencies to identify other passengers who may have been exposed and determine whether they need to isolate or quarantine.
“Measles is one of the most contagious diseases we see, which is why identifying potential exposures quickly is so important,” county Health Director Dr. Kanika Cunningham said.
The case comes as measles infections climb across the country. The Centers for Disease Control and Prevention reported 3,471 confirmed cases nationwide as of Sept. 17, compared with 2,289 during all of 2025.
Missouri’s vaccination rate among children entering school was 89.5%, below the 95% level needed to help prevent community outbreaks, according to a state health alert.
Illinois also has seen an increase, with 22 measles cases reported this year compared with 14 during all of 2025, according to the Illinois Department of Public Health.
Although the St. Louis County resident was fully vaccinated, such infections are uncommon. Two doses of the measles, mumps and rubella vaccine are approximately 97% effective at preventing measles, according to the CDC.
County Executive Sam Page urged families to make sure their children are current on their recommended vaccinations and encouraged adults with questions about their immunity to consult their doctors.
Measles spreads through the air when an infected person breathes, coughs or sneezes. The virus can remain in an enclosed space for up to two hours after the person leaves.
Symptoms generally appear seven to 14 days after exposure but can take as long as 21 days to develop. They typically begin with a high fever, cough, runny nose and red or watery eyes. Small white spots may appear inside the mouth before the characteristic measles rash develops.
Cunningham urged residents to check their vaccination records and contact their health care provider if they develop symptoms.
“We encourage residents not to panic,” she said.
Anyone who believes they may have been exposed should contact a health care provider for guidance. Those who develop symptoms should call before visiting a doctor’s office, urgent care center or emergency department so medical staff can take precautions to protect other patients.
St. Louis County residents with questions can call the Department of Public Health at 314-615-1630 during regular business hours or 314-615-7677 after hours.