February through April 2024 Chicago faced a measles outbreak within a migrant shelter, prompting action from the CDC.
3D illustration of the highly contagious measles on somebody's back.
Image credits: Unsplash
“Measles is the most contagious and easily transmittable virus we know—far more than COVID or influenza,” said Patricia A. Stinchfield, RN, MS, CPNP, president, National Foundation for Infectious Diseases (NFID).1
This outbreak resulted in 57 confirmed cases, including residents, staff, and linked community members. The CDC employed dynamic disease modeling to forecast outbreak trajectories and evaluate public health interventions' efficacy. Real-time, dynamic disease models proved invaluable in guiding public health responses, aiding in setting expectations regarding outbreak size and duration and quantifying the effectiveness of interventions.
As of April 8, the model projected a median final outbreak size of 58 cases, with an interquartile range of 56–60 cases. The counterfactual analysis demonstrated the significant impact of early deployment of public health interventions, with mass vaccination and active case-finding reducing the likelihood of an outbreak exceeding 100 cases from 69% to 1%.
“If mass vaccination had been delayed by 1 week, there would have been an eightfold increase (from 1% to 8%) in the chance of an outbreak of 100 or more cases over the scenario in which mass vaccinations were deployed on March 8," according to the CDC. "These results are consistent with those from the OAW response, in which a modeled 7-day delay in vaccination would have yielded a 50% increase in median outbreak size.”2
CDC employed a dynamic disease model, continuously updated as new cases emerged, to simulate the measles outbreak among shelter residents. This model allowed real-time forecasting of outbreak progression and enabled the assessment of various intervention strategies. A counterfactual analysis to compare outbreak scenarios with and without interventions, focusing on mass vaccination and active case-finding.
“This parameter emphasized the importance of active case surveillance because of the possibility of many secondary cases," according to the CDC. "CDPH used these findings, along with observed measles cases among shelter residents who had received a single dose of MMR vaccine, to demonstrate the need for continued vaccination campaigns during the outbreak, including a second dose campaign 28 days after the first dose, and to recommend that shelter staff members and essential visitors provide evidence of immunity to measles.”2
Despite its utility, the modeling approach had limitations. Parameters were subject to substantial uncertainty due to the outbreak's limited size. Additionally, the model structure and assumptions may have influenced the accuracy of projections. The model did not account for the relocation of 22 family units to a quarantine hotel, potentially affecting final outbreak size estimations.
In a recent report by the CDC, at the end of 2023, US measles elimination status was maintained due to the absence of sustained measles virus transmission for 12 consecutive months and the presence of a well-performing surveillance system. Despite this, the risk for widespread US measles transmission remains low due to high population immunity. The increase in cases during the first quarter of 2024 indicates that additional activities are needed to enhance routine measles, mumps, and rubella vaccination coverage, particularly among close-knit and under-vaccinated communities. These activities should include promoting vaccination before international travel and rapidly investigating suspected measles cases.2
During the specified period, the CDC was notified of 338 confirmed measles cases. Notably, 97 (29%) of these cases occurred during the first quarter of 2024, representing a more than seventeenfold increase over the mean number of cases reported during the first quarters of 2020–2023. Among the reported cases, the median patient age was 3 years, with ages ranging from 0 to 64 years. Most patients, 309 (91%), were unvaccinated or had unknown vaccination status. Additionally, 336 case investigations included information on ≥80% of critical surveillance indicators. From 2020–2023, the longest transmission chain lasted 63 days. 3
The findings underscore the role of proactive public health measures, such as prompt mass vaccination and active case-finding. While the study provides valuable insights, it also highlights the need for continued refinement and validation of modeling approaches in outbreak response.
