A recent measles outbreak in the Chicago area last fall, detailed in a report from Public Health Alerts, involved a preschooler who contracted the virus and subsequently transmitted it to two vaccinated adults. One or both of these adults then spread the infection to another individual who was unvaccinated.
During this outbreak, at least 99 people were exposed to measles. A significant portion, 79 individuals (80%), were exposed in emergency department waiting rooms on two separate occasions in Cook County, Illinois. Public health officials noted that incomplete information gathered during the investigation may have contributed to preventable exposures within healthcare settings.
The report’s authors suggested that the presence of Immigration and Customs Enforcement (ICE) operations in the area might have influenced the difficulty in obtaining comprehensive exposure details.
Public Health Alerts, a joint initiative by NEJM Evidence and CIDRAP, aims to provide reliable, expert-reviewed reports that translate frontline observations into actionable public health evidence, addressing a need for robust data. An accompanying editorial in NEJM Evidence further elaborates on this initiative.
No Household Members Infected
On September 13, 2025, the Cook County Department of Public Health (CCDPH) was alerted to a suspected measles case involving a 4-year-old unvaccinated child. This child, the index patient, had arrived at O’Hare International Airport on September 11 from a country where measles is endemic, experiencing cough and fever during the flight. After developing a facial rash on September 12, the child was taken to an emergency department (ED) the following day. Measles was confirmed by Illinois Department of Public Health (IDPH) testing on September 15.
Among the 29 ED contacts identified, 18 (62%) had documented immunity. Seven individuals received postexposure prophylaxis with the measles, mumps, and rubella vaccine (MMR PEP) due to unconfirmed vaccination status or antibody titers. Six contacts had unknown immune status, with four not responding to inquiries and two ceasing communication after initial contact. Notably, the index patient’s household, consisting of three siblings and two parents, all had documented immunity to measles and did not contract the virus.
Later, on September 28, a fully vaccinated 49-year-old woman presented to the same ED with a fever and rash. IDPH testing confirmed measles the next day. While not initially identified as a direct contact of the index patient (only her sister was), she had been present in the ED on September 13 and was exposed. Fifty ED contacts were identified in relation to this second case. Of these, 36 (72%) were immune, seven received MMR PEP, and seven had unknown immune status. Eleven coworkers and two young children were also exposed, none of whom developed measles.
However, the sister of this vaccinated patient developed symptoms on October 3, twenty days after her exposure to the index patient. Her unvaccinated child subsequently tested positive for measles on October 15. This child sought care at a pediatric ED, but due to prior alerts from CCDPH, no further exposures occurred at that facility.
Concerns Over ICE Presence
The report concludes that the difficulties in obtaining complete exposure information led to a potentially avoidable secondary exposure in the first hospital’s ED. The authors stated that during interviews, references were made to two additional household contacts who could not be identified or interviewed by CCDPH staff.
The report also noted that "Operation Midway Blitz," an ICE enforcement initiative, was active in the Chicago area from September 8 and overlapped with the outbreak period. Several family members expressed concerns about privacy due to these ongoing ICE activities, which the authors speculate may have contributed to the challenges in identifying all potentially exposed individuals.