A measles outbreak in southwestern Wisconsin has grown to 161 confirmed and probable cases, prompting health officials to warn northern Illinois residents who traveled to the region — including over Labor Day weekend — to be aware of the risk of exposure.
The Wisconsin Department of Health Services reported 155 confirmed and six probable measles cases as of Friday in Grant, Iowa and Lafayette counties. The agency said its investigation indicates measles is continuing to spread locally in the three-county area. There have been 163 measles cases statewide in Wisconsin this year.
The outbreak is of particular concern to Illinois health officials because southwestern Wisconsin is a popular destination for residents of northern Illinois, health officials said this past week.
The Kane County Health Department specifically warned residents about the outbreak in a Wednesday health advisory.
"KCHD encourages residents who traveled to southwestern Wisconsin to be aware of the ongoing outbreak and to monitor for symptoms of measles, particularly if they are not vaccinated or are unsure of their vaccination status," the health department said.
The warning comes as Kane County investigates its second measles case of 2026.
The second case was identified in a vaccinated individual who was a contact of Kane County's first case, which was reported Aug. 24. Potential exposure locations included Kaneland High School in Maple Park and Next Generation Pediatric Clinic in South Elgin.
The Kane County Health Department said it is notifying people known to have potentially been exposed and providing guidance based on their vaccination and exposure status.
The breakthrough case highlights that while the measles, mumps and rubella vaccine provides strong protection, vaccinated people can develop measles after exposure — though it is rare, health officials said. People who develop measles despite vaccination generally have milder illness and are less likely to experience serious complications or transmit the virus.
Two doses of the MMR vaccine provide approximately 97% protection against measles, according to Kane County.
The first Kane County case demonstrated how quickly a single infection can create a significant public health response. According to the health department, that case resulted in as many as 1,600 potential exposures requiring public health officials to identify and notify people who may have been exposed.
Illinois also continues to monitor measles activity. The Illinois Department of Public Health reported 15 measles cases statewide as of Friday. and has identified several recent potential exposure locations, including Kaneland High School in Maple Park, a pediatric clinic in South Elgin and Chicago Union Station.
Measles is highly contagious and spreads through the air when an infected person coughs or sneezes. The virus can remain in the air for up to two hours after an infected person leaves an area.
Symptoms can include:
Health officials are advising anyone who may have been exposed to monitor for symptoms for 21 days after their last potential exposure.
People who develop symptoms should stay home and call their health care provider before going to a medical facility. Health officials specifically caution against walking into an emergency department or doctor's office without calling ahead because doing so could expose other patients and health care workers.
People who are unvaccinated and have been identified as contacts of a confirmed case may be placed under quarantine and should follow instructions from public health officials.
The Wisconsin outbreak is also being monitored through wastewater surveillance.
Wisconsin DHS reported recent measles virus detections in wastewater samples from Iowa County on Aug. 26 and Aug. 31, Grant County on Aug. 19 and Lafayette County on Aug. 25. DHS said wastewater detections indicate that one or more people infected with measles were present in the community around the time samples were collected, although the results cannot determine how many infected people were present.
Officials are urging residents to check their vaccination records, particularly if they recently traveled to southwestern Wisconsin or are unsure whether they have received both recommended MMR doses.
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