Measles in a Chicago Migrant Shelter, February–May 2024

Measles is a highly contagious respiratory virus that can cause severe complications, hospitalization and death. It was declared eliminated from the United States in 2000, but it keeps arriving from abroad. One dose of measles vaccine is 93% effective at preventing it, two doses 97%.

The setting

Since August 2022, about 41,000 migrants had arrived in Chicago from the southern border, 88% of them from Venezuela, where routine childhood vaccination, including against measles, has fallen — first-dose coverage there dropped from 96% to 68% during 2017–2021. On February 22, 2024, about 12,000 people lived in 27 city-run temporary shelters. The largest, shelter A, housed about 2,100 people with shared sleeping areas, dining room and bathrooms; some rooms held 500 or more.

The first case

A 1-year-old boy living in shelter A developed a rash on February 26 and was admitted to hospital the next day with suspected measles. The Chicago Department of Public Health (CDPH) was notified only on March 4. He had arrived in the country more than 5 months earlier, had had one dose of MMR vaccine 5 weeks before the rash, and had no recent travel or known exposure. On March 7 testing confirmed wild-type measles (a special test, MeVA, distinguishes vaccine virus from the real thing — up to 5% of MMR recipients get a vaccine rash that looks just like measles). CDPH warned residents and staff that evening. Because the shelter was so crowded, everyone who had been inside during his infectious period, February 22–27, was treated as exposed.

The outbreak

From February 26 to May 13, CDPH confirmed 57 cases linked to shelter A: 52 residents, 3 staff and 2 people in the community.

• Age: median 3 years (range 0–52); more than half were between 6 months and 4 years old.
• Origin: 84% of those with a known country were from Venezuela; they had arrived a median of 124 days before their rash.
• Vaccination: 41 (72%) had no record of measles vaccine. Sixteen (28%) had at least one dose 21 or more days before first exposure, and 4 had two doses; the median age of these vaccinated patients was 9.5 years, and 7 were under 5.
• Illness: 51 (89%) were hospitalized, for isolation, complications or both; nobody died.
• Virus: 52 specimens yielded identical genotype D8 sequences; 5 could not be sequenced.
• Two cases were in people who had moved on March 7 or later to less crowded shelters with private rooms; nobody caught measles at those shelters. The last known exposure at shelter A was April 5.

Cases by rash onset date, and the response. CDC.

The response

CDPH worked with state and local health departments, hospitals, city agencies and shelters, and relied on trusted community health workers and leaders — including Spanish-speaking promotores de salud — to explain measles, vaccination and quarantine.

• Vaccination, March 8–10. Within a day of confirmation, staff checked paper records and the state immunization registry and offered MMR vaccine to every non-pregnant resident 6 months or older without a record, and a second dose to those 1 year or older whose first dose was at least 28 days earlier. Records were verified for 784 (44%) of the 1,801 residents and 882 (49%) were vaccinated, so by March 11, 1,666 (93%) had at least one documented dose. By May 13 CDPH had run about 130 vaccination events in 25 shelters and given about 9,500 MMR doses, prioritizing shelters that had taken in shelter A residents and those with pregnant women and young children, with a second-dose campaign at shelter A on April 8–10.
• Case-finding. From March 8 medical and shelter staff went bed to bed looking for fever, rash, cough, runny nose or red eyes. People who were ill were tested on site or taken straight to hospital; isolation came a median of 1 day after rash onset.
• Quarantine. Residents without a dose at least 21 days before exposure were asked to stay in the shelter for 21 days after vaccination (or after last exposure, if unvaccinated). Quarantine was voluntary, to keep residents' cooperation. Children in quarantine stayed out of school from March 8. Twenty-two families with the most vulnerable members — babies 6 months or younger, pregnant women without immunity, people with weak immune systems — were moved to a hotel on March 11–12. New intake stopped on March 8, and only residents with a documented dose could move to other shelters. Because the shelter had no way to isolate sick people, confirmed and strongly suspected cases were isolated in Chicago hospitals.

Lessons

• Speed of notification. MMR vaccine prevents measles only if given within 72 hours of exposure. The 6-day delay before CDPH heard of the first case meant residents had missed that window; the mass vaccination still likely prevented infections from later exposures and shortened the outbreak.
• Isolation space. With nowhere to isolate people at the shelter or elsewhere in the community, patients without a medical need filled hospitals' airborne-infection isolation rooms. Dedicated isolation space outside hospitals is needed.
• Vaccinated cases. A larger share of cases were in vaccinated people than in recent national data, probably because exposure in crowded living was so intense. Primary vaccine failure — no immune response — happens in about 4% of people after one MMR dose and rarely after two; secondary failure, infection despite a response, is linked to prolonged or close exposure and has been seen in crowded settings. Which applies here was still being studied.
• Local spread. Because the first patient had been in Chicago for months, he probably caught measles locally, though no link to another case was found.

High MMR coverage among both long-time and newly arrived residents is what keeps measles from spreading within and beyond shelters.

Sources

• Kimberly Gressick, Amy Nham, Thomas D. Filardo and colleagues, with the Chicago Department of Public Health Measles Response Team, "Measles Outbreak Associated with a Migrant Shelter — Chicago, Illinois, February–May 2024," MMWR 73(19), 2024. https://www.cdc.gov/mmwr/volumes/73/wr/mm7319a1.htm
• CDC, MMR vaccine recommendations: https://www.cdc.gov/vaccines/vpd/mmr/hcp/recommendations.html