A Public Health Alerts report today describes a small measles outbreak last fall in the Chicago area involving a preschooler who transmitted the disease to two vaccinated adults, one or both of whom transmitted the virus to another, unvaccinated person.
At least 99 people were exposed to measles during this outbreak, including 79 (80%) exposed in emergency department (ED) waiting rooms on two separate dates in Cook County, Illinois. The authors of the report, from the Cook County Department of Public Health (CCDPH) and the Illinois Department of Public Health (IDPH), say that investigators into the outbreak were able to collect only incomplete information, which might have led to preventable exposures in healthcare settings.
The authors of the report speculate that the presence of Immigration and Customs Enforcement officers in the area might have played a role in the inability to obtain complete exposure information.
Public Health Alerts, a collaboration between NEJM Evidence and CIDRAP, fills a gap in reliable data, offering expert-reviewed reports that translate frontline observations into actionable public health evidence. An NEJM Evidence editorial explains the initiative further.
No household contacts of index patient contracted measles
The CCDPH received notification on September 13, 2025, of a suspected measles case in an unvaccinated 4-year-old child (patient A, or the index patient) who had arrived at O’Hare International Airport on September 11 from a country with endemic measles spread. The child had a cough and fever on board the flight.
After developing a facial rash on September 12, the child was brought to an ED the following day. IDPH testing confirmed measles on September 15.
Officials identified 29 ED contacts from the ED. Overall, 18 (62%) were immune, five (17%) received postexposure prophylaxis (prevention) with the measles, mumps, and rubella vaccine (MMR PEP) on September 16 because they were unable to provide proof of vaccination or positive titer results, and six (21%) had unknown immune status, including four who did not respond and two whom the authors reached but did not respond afterward.
The infected child lived with three siblings and two parents, all with documented immunity to measles. None of them contracted measles.
On September 28, however, a 49-year-old fully vaccinated woman (patient B) came to the same ED with a fever and rash; IDPH testing confirmed measles the next day. Although the previous investigation did not identify the woman as a contact of the index patient—only her sister—she was indeed present in the ED on September 13 and exposed to the infected child.
The challenges experienced ascertaining complete exposure information resulted in a potentially avoidable second exposure.
Fifty ED contacts were identified as exposed to patient B. In total, 36 (72%) were immune, seven (14%) received MMR PEP, and seven (14%) had unknown immune status. Additional exposures included 11 coworkers and two young children. None of these contacts tested positive for measles.
Patient B’s sister, however developed a fever and rash on October 3, 20 days after exposure to the index patient. Her unvaccinated child tested positive for measles on October 15. This child sought care at a pediatric ED, but, because, the CCDPH had alerted hospital staff, no exposure occurred at this second ED.
ICE presence may have played a role
The authors conclude, “The challenges experienced ascertaining complete exposure information resulted in a potentially avoidable second exposure in hospital A’s ED. During interviews with patients B and C and several of their household contacts, references were made to two additional household contacts, but CCDPH staff were unable to ascertain their names or to interview them.”
The add, “‘Operation Midway Blitz,’ the name given to Immigration and Customs Enforcement’s (ICE’s) immigration enforcement activities in the Chicago area, began on September 8 and were underway during this outbreak. Several family members informed CCDPH staff that they had concerns regarding privacy, given ongoing ICE activities.
“We speculate these concerns may have contributed to the challenges identifying all potentially exposed individuals.”
