Excess Pneumonia Mortality During a 2014-2015 Legionnaires' Disease Outbreak in Genesee County, Michigan

Background: From June 2014 to October 2015, the Michigan Department of Health and Human Services reported an outbreak of 90 cases of Legionnaires' disease, including 10 deaths, in Genesee County, Michigan. As Legionnaires' disease is not routinely tested for as a cause of community-acquired pneumonia, we hypothesized that the size of the outbreak was underestimated. Methods: We used Centers for Disease Control and Prevention (CDC) Wide-ranging ONline Data for Epidemiologic Research data to compare pneumonia mortality in Genesee to similar counties from 2011 to 2017. We used data from the Genesee County Vital Records Division to assess geographic overlap of pneumonia mortality with reported Legionnaires disease cases by census tract. Results: We estimated 70.0 excess pneumonia deaths (90% uncertainty interval: 36-103) in Genesee County during the outbreak. Areas of high pneumonia mortality overlapped with those with high Legionnaires' disease incidence. Conclusions: These findings are consistent with the hypothesis that the Legionnaires' disease outbreak was larger than reported. Earlier outbreak detection and response may have facilitated identification of additional cases.