Background: Mathematical models suggest that social distancing measures, such as school closures, may mitigate community transmission during an influenza pandemic. Because closures are disruptive to schools and families, they are rarely employed during seasonal influenza outbreaks. A rare circumstance enabled us to examine the association between school closure and absenteeism during a seasonal influenza outbreak when half of King County, Washington public schools closed for a winter recess 19-23 February 2007, while half remained open for all or part of the week.
Methods: Using absenteeism as a proxy for influenza activity, we tested the hypothesis that schools on break would experience lower rates of post-break absenteeism than schools remaining open. We conducted daily retrospective and prospective surveillance from 5 February-9 March 2007 in schools on break (n = 256) and in session (n = 205). We use generalized estimating equations with Poisson distribution to evaluate whether mean absenteeism after the break differed between schools on break and those in session, adjusting for baseline absenteeism and repeated measurements by schools over time.
Results: Results indicate no difference in post-break absenteeism in schools on break compared with schools that remained in session (relative risk = 1.07 [95% confidence interval = 0.96-1.20]). This result held in elementary schools (1.00 [0.91-1.10]), where absenteeism patterns are thought to be most representative of community influenza activity.
Conclusion: We did not find that school closure during a seasonal influenza outbreak reduced subsequent absenteeism. However, limitations in this "natural experiment" hampered our ability to detect a benefit if one truly was present.