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

Abstract

In April 2009, following the first school closure due to 2009 pandemic influenza A (H1N1) (pH1N1) in Chicago, Illinois, area hospitals were inundated with patients presenting with influenza-like illness (ILI). The extent of disease spread into the surrounding community was unclear. We performed a household survey to estimate the ILI attack rate among community residents and compared reported ILI with confirmed pH1N1 cases and ILI surveillance data (ie, hospital ILI visits, influenza testing, and school absenteeism). The estimated ILI attack rate was 4.6% (95% confidence interval, 2.8%-7.4%), with cases distributed throughout the 5-week study period. In contrast, 36 (84%) of 43 confirmed pH1N1 cases were identified the week of the school closure. Trends in surveillance data peaked during the same week and rapidly decreased to near baseline. Public awareness and health care practices impact standard ILI surveillance data. Community-based surveys are a valuable tool to help assess the burden of ILI in a community.

Figure 1.

Correlation of epidemiologic indicators of 2009 pandemic influenza A (H1N1) (pH1N1) virus activity in community A, from 13 April through 17 May 2009. A, Influenza-like illness (ILI) cases identified through the community survey by illness onset date. B, Confirmed cases of pH1N1 in community A, by specimen collection date (bars) and illness onset date (line). C, Number of ED and outpatient visits for ILI for the 3 hospitals servicing community A, by day. D, Proportion of students absent from school X in community A, by day. E, Proportion of students absent from the 4 other elementary schools in community A, by day.