Excess Pneumonia Mortality During a Legionnaires’ Disease Outbreak in Flint, Michigan

Introduction From June 2014-October 2015, the Michigan Department of Health and Human Services (MDHHS) reported a Legionnaires’ disease (LD) outbreak in Genesee County, Michigan, of which Flint is the county seat. MDHHS’ final case count for the outbreak was 90 cases, including 10 deaths among Genesee County residents. As LD is not routinely tested for as a cause of community-acquired pneumonia, the size of the outbreak may have been underestimated. Specifically, some LD cases may have been classified as pneumonias of other or unexplained etiologies. Objective To estimate excess pneumonia deaths in Genesee County during the 2014-15 LD outbreak and compare this with reported deaths due to LD. Methods We used data from the CDC WONDER database, which contains monthly county-level counts of death certificates, on all pneumonia deaths among residents of Genesee and a set of similar counties from 2011-2017. We calculated excess pneumonia deaths in Genesee during the period of the LD outbreak (May 2014-October 2015). We used data from the Genesee County Vital Records Division to map pneumonia deaths by census tract to assess geographic overlap with reported LD cases. Results We estimated 70.0 excess pneumonia deaths (90% uncertainty interval (UI): 36 to 103) in Genesee County during the LD outbreak. This is substantially higher than the 10 LD deaths among Genesee County residents reported by MDHHS. Areas of high pneumonia mortality overlapped with those with high LD incidence and were primarily located in western Flint and northwestern non-Flint Genesee County. Conclusions These findings are consistent with the hypothesis that the LD outbreak was larger than reported. Earlier detection and response to this outbreak may have facilitated identification of these additional cases.