Influenza-like illness in a community surrounding a school-based outbreak of 2009 pandemic influenza A (H1N1) virus – Chicago, Illinois, 2009

Background: In April 2009, following the first outbreak of pandemic H1N1 (pH1N1) in a Chicago school, area hospitals were overwhelmed with patients with influenza-like illness (ILI). However, it was unknown how much pH1N1 had spread into the surrounding community. Methods: We performed a household survey to estimate the ILI attack rate among community residents, and compared reported ILI to confirmed pH1N1 cases, hospital surveillance data, and school absenteeism. Results: The estimated ILI attack rate in the community was 4.6% (95% CI, 2.8% to 7.4%), with cases distributed throughout the assessed 5-week period. In contrast, 36 (86%) of 42 confirmed pH1N1 cases were identified in the week following the local school closure. Other ILI surveillance data peaked during the same week and rapidly declined to near baseline. Living in higher density housing and attendance/work at a primary school were independent risk factors for ILI. Conclusions: Despite a school-based pH1N1 outbreak, the ILI attack rate in the surrounding community remained relatively low. Hospital ILI visits, influenza testing, and absenteeism at other area schools peaked after the affected school was closed, but did not reflect patterns of reported ILI among community residents. Public awareness of pH1N1 impacts healthcare utilization and school attendance.