Notes from the Field: Measles Outbreak — Cook County, Illinois, October–November 2023

Investigation and Outcomes

During the child’s October 5–11 health care encounters, 247 health care workers^† and 177 patients and patient companions^§ were considered to have been exposed, including 13 children aged <1 year, five immunosuppressed children, and one child aged >1 year with no history of MMR vaccination. Among these 19 children, two received a dose of MMR vaccine within 72 hours of the exposure, and 13 received immune globulin.

The index patient’s household contacts included two siblings with no history of MMR vaccination and with serologic testing indicating measles susceptibility. One sibling, aged 4 years, (patient B) arrived in the United States at the same time as the index patient (September 29). The second sibling, aged 9 years, (patient C) had arrived in the United States in January 2023. Both siblings developed measles while in quarantine with rash onsets on October 22 (patient B) and November 1 (patient C). Patient B also reported fever, cough, coryza, and conjunctivitis; patient C also reported fever. Neither child was hospitalized, although patient B required an ED visit at hospital A for supportive care. On October 17, exposure notification letters were delivered to all residents in the apartment building where the index patient lived.

On October 30, hospital A notified CCDPH of another child, aged 2 years, (patient D) who had been evaluated in an ED early that morning with fever, cough, and coryza, then discharged. The family of that child lived in the same 2-story apartment building as the index patient, but on a different floor. Patient D had no history of MMR vaccine; the child’s parents reported objections to MMR vaccine based on personal beliefs and perceptions about vaccine side effects. Measles was confirmed in this child by RT-PCR testing on October 30; rash onset occurred on November 1. The families of patients A–C and patient D had different cultural backgrounds from one another and spoke different primary languages; both families independently reported no contact with the other family. Their apartment units did not have shared ventilation; however, laundry facilities and building entrances were shared.

On October 31, testing was also performed for a sibling of patient D, a child aged 1 year (patient E), also with no history of MMR vaccine, who had isolated coryza and who attended a child care facility^¶ on October 30 while symptomatic; a nasopharyngeal swab collected in the home confirmed measles by RT-PCR testing. Attendees and staff members of the child care facility were notified the same day. One child aged 2 months received immune globulin, one child aged 11 months received 1 dose of MMR vaccine, and 11 children who had received their first MMR vaccine dose received an early second dose as post-exposure prophylaxis.** Fever in patient E did not occur until November 6, and rash did not appear until November 9, which was 9 days after the positive RT-PCR test result and child care facility notification.^†† Measles testing is indicated for susceptible contacts of measles cases when the contact has prodromal symptoms (i.e., fever, cough, coryza, or conjunctivitis); however, isolated coryza experienced by this patient at the time of specimen collection might not have been related to measles. Because testing for measles before fever onset is not typically performed, an accurate infectious period for this patient was difficult to ascertain. Patient E’s symptoms resolved without requiring emergency care or hospitalization. This activity was reviewed by CDC, deemed not research, and was conducted consistent with applicable federal law and CDC policy.^§§