The effect of reactive school closure on community influenza-like illness counts in the state of Michigan during the 2009 H1N1 pandemic.

BACKGROUND In sum, 559 Michigan schools were closed as a nonpharmaceutical intervention during the influenza A 2009 (H1N1) pandemic. METHODS By linking the proportion of schools closed within a district to state influenza-like illness (ILI) surveillance data, we measured its effect on community levels of ILI. This analysis was centered by the peak week of ILI for each school district, and a negative binomial model compared three levels of school closure: 0%, 1%-50%, and 51%-100% of schools closed from three weeks leading up to ILI peak to four weeks following ILI peak rate. RESULTS We observed that school closures were reactive, and there was no statistically significant difference between ILI rates over the study period. There was an elevated rate ratio for ILI at 51%-100% closure, and a reduction in the rate ratio at the 1%-50% compared to the 0% closure level. CONCLUSIONS These findings suggest that district level reactive school closures were ineffective.