Case study shows measles outbreak at Chicago shelter was mostly among unvaccinated people

a study Published yesterday Morbidity and Mortality Weekly Report (MMWR) Details about the measles outbreak that sickened 57 people at a Chicago immigrant shelter this year and how it happened. A rapid and coordinated mass vaccination campaign helped contain the infection.

Measles is a highly contagious respiratory virus that can cause serious complications, hospitalization, and death. Although the United States declared measles eradicated in 2000, cases continue to occur due to the continued global spread and vaccine shortages, a Centers for Disease Control and Prevention (CDC)-led research team said. “One-dose and two-dose measles vaccines are 93% and 97% effective in preventing measles, respectively.”

They say that since August 2022, about 41,000 immigrants have arrived in Chicago from the U.S. southern border, 88% of whom are from Venezuela, and 96% of them have received routine childhood immunizations compared to 2017. % to 68% in 2021. Measles vaccine.

The first reported case was a 1-year-old boy

On February 22, 2024, approximately 12,000 people were staying in 27 temporary migrant shelters run by the city of Chicago. On that day, the largest of the shelters, referred to in the report as “Shelter A,” housed about 2,100 people, with some rooms holding more than 500 people. In addition to communal sleeping areas, the migrants also shared dining areas and bathrooms.

Given the congregate settings, high rates of measles transmission, and low measles vaccination rates among shelter residents, the measles virus is likely to have spread among the approximately 2,100 exposed shelter residents. It had the potential to spread rapidly.

The first case of the outbreak was confirmed to be a 1-year-old boy living in an evacuation center. He developed a rash on February 26 and was hospitalized the next day with suspicion of measles.

The boy had arrived in the United States more than five months ago and had received one dose of measles, mumps, rubella (MMR) vaccine five weeks before the onset of symptoms. He had not traveled recently or been exposed to measles. The Chicago Department of Public Health (CDPH) confirmed the infection on March 7th.

“Given the congregate settings, high rates of measles transmission, and low measles vaccination rates among shelter residents, measles virus is present among approximately 2,100 exposed shelter residents. “It has the potential to spread rapidly,” the researchers wrote.

CDPH worked with state and local health departments, medical facilities, city officials, and shelters to investigate and respond to the outbreak. On March 8, CDPH began case-finding efforts and began a mass vaccination campaign at the shelter, vaccinating 882 residents over three days and confirming previous vaccinations for 784.

“These activities resulted in measles vaccination coverage (defined as one or more measles vaccinations) of 93% by March 11,” the authors said. “Through May 13, a total of 57 confirmed measles cases were reported to be associated with residents of Shelter A or contact with people in Shelter A.”

As of May 13, CDPH has held approximately 130 mass vaccination events at 25 immigrant shelters in Chicago, including shelters that previously received residents from Shelter A and shelters housing pregnant women and young children. Approximately 9,500 doses of MMR vaccine were administered with priority given to “This strategy included an additional vaccination event in Shelter A starting March 25th and an intensive second vaccination campaign from April 8th to 10th,” the study said. said the people.

The index infection was likely locally transmitted.

Most infections (72%) occurred in unvaccinated populations, and 28% were diagnosed in residents who had received at least one measles vaccination at least 21 days before first known measles exposure. It was done.

The risk of infection within and outside of shelters can be reduced by maintaining high MMR vaccination rates among both existing and newly arrived residents.

Of the 57 infected people, 52 were residents of evacuation centers, three were staff members, and two were local residents. Median patient age was 3 years. The two cases occurred among people who lived in affected shelters but were resettled or moved to less crowded shelters with dedicated sleeping areas after March 7. No secondary infections were confirmed at these evacuation centers.

Fifty-one people (89%) were hospitalized due to isolation or measles complications, but no deaths were reported. The last confirmed case in the affected shelters was April 5. As of May 13, the same measles genotype D8 sequence had been identified in 52 case specimens. The remaining five isolates could not be sequenced.

”Although members of the communities affected by this outbreak recently arrived in the United States, the index patient arrived in Chicago several months before becoming ill, suggesting that the disease was locally transmitted.” concluded. Maintaining high MMR vaccination rates among both existing and newly arrived residents can help alleviate this problem. ”