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Girl getting vaccine

Study: Mass vaccination helped mitigate measles outbreak in Chicago migrant shelter

May 16, 2024

Chicago health officials moved quickly to investigate potential measles cases in a migrant shelter and vaccinate residents, which likely helped prevent a worse outbreak, according to two new reports.

“This outbreak underscores the need to ensure high vaccination coverage among communities residing in congregate settings,” authors wrote in a new Morbidity and Mortality Weekly Report.

Health officials from the Centers for Disease Control and Prevention (CDC), Chicago and Illinois looked at the response to the 57-case outbreak that started Feb. 26 when a 1-year-old boy in a temporary migrant shelter in Chicago developed a rash. He was hospitalized a day later, and measles was confirmed March 7.

The day after confirmation of the case, Chicago health officials implemented a mass vaccination campaign for those who had not received measles, mumps and rubella (MMR) vaccine. They worked with community partners fluent in Spanish who served as liaisons to the residents. The CDC also supported the efforts.

The team was able to verify previous vaccination for 783 of the 1,801 residents and vaccinate 882 within three days, bringing the vaccination rate with at least one dose to 93%.

Health officials also screened residents for symptoms. Some symptomatic residents were tested on site, while others were transported to a hospital for testing and isolation.

Shelter residents with confirmed measles cases isolated in Chicago hospitals, which study authors noted put a strain on these facilities. They said this highlights “the importance of having dedicated isolation space outside of hospitals for patients without medical need.”

Officials recommended that residents who had not received a measles vaccine at least 21 days before exposure quarantine in the shelter and stay out of school, although this was not mandatory. The shelter stopped taking new residents on March 8, and families with high-risk members transferred to a repurposed hotel for quarantine.

As of May 13, there have been 57 confirmed measles cases associated with the shelter. The median age is 3 years and most of the patients were from Venezuela, a country that has experienced declining childhood immunization rates. About 72% of the cases were among people who were unvaccinated, 21% had one dose of MMR vaccine and 7% had at least two doses. Authors said cases among people who had been vaccinated were higher than expected, likely due to people living so close together in the shelter.

Authors said with their early intervention, modeling shows just a 1% chance of an outbreak of 100 or more cases. That chance would have risen to 8% if mass vaccination had been delayed a week and 69% with no intervention, according to one of the studies. However, they also noted the mass vaccination could have started even earlier if six days hadn’t passed between the initial patient being hospitalized and the time Chicago officials were notified.

As of May 13, Chicago had led about 130 mass vaccination events across 25 migrant shelters and administered about 9,500 doses of MMR vaccine.

Nationwide, there have been 132 cases of measles this year in 21 jurisdictions. While the virus was declared eliminated in 2000, CDC experts have said the spike this year may threaten that status. They are encouraging routine vaccination coverage, vaccination before international travel and swift identification of possible cases.

When considering measles, isolate the patient, notify the state, tribal, local or territorial health department, follow CDC testing recommendations and manage the patient in coordination with local or state health departments including appropriate post-exposure prophylaxis.




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