Purpose: Control of pandemic influenza by social distancing measures such as school closures is a major theme of pandemic planning. However, the extent to which these measures actually impact the progression of a pandemic has never been established. Two waves of pandemic influenza (pH1N1) were observed in North America in 2009. A unique opportunity to identify and quantify the causes of the large reduction in incidence between the two waves is presented by the Canadian province of Alberta, which conducted widespread, unrestricted virological testing continuously until the middle of the second wave in October.
Method: We examined the complete database of 35,510 specimens submitted for virological testing to the Alberta Provincial Laboratory for Public Health from 19 April 2009 to 2 January 2010, which included both waves of the pH1N1 pandemic. We compared the pH1N1 age-structured incidence pattern with weather time series and the pattern of closing and opening of schools. We used mathematical models to investigate the mechanisms that could have given rise to the observed incidence patterns.
Result: The data strongly indicate that the ending and restarting of school terms had a major impact in attenuating the first wave and sparking the second. Mathematical models suggest that school closure reduced transmission among schoolchildren by more than 50% (95% CI 52–100%), and that this was a key factor in interrupting transmission.
Conclusion: Unrestricted virological testing during an influenza pandemic has made it possible to provide unequivocal evidence that closing schools can have dramatic effects on transmission of pandemic influenza. School closure appears to be a potentially effective strategy for slowing the spread of pandemic influenza in countries with social contact networks similar to those in Canada.