How spread of the influenza A(H1N1)pdm09 pandemic was affected by school closures – Alberta, CanadaArchived

Effects of School Closure on Incidence of Pandemic Influenza in Alberta, Canada Earn DJD, He D, Loeb MB, et al. Annals of Internal Medicine, February 7, 2012; vol. 156 no. 3 173-181

The objective of this study was to investigate the extent to which school closures  affected the progression of 2009 A(H1N1)pdm09 pandemic. The authors examined correlations between school closures and the major confounder weather changes and the incidence of influenza A(H1N1) infections and hence inferred influenza transmission in Alberta, Canada. There schools closed routinely for the summer holidays at the end of June and restarted in September. This was in the middle of a Spring – Summer wave of pandemic transmission. Based on these results of extensive diagnostic testing the methodology fitted mathematical transmission models to data that compared the pattern of confirmed influenza A(H1N1) cases with the school calendar and weather patterns. The data included in this analysis were obtained from: 2009 virological test results, 2006 census data, 2009 daily temperature and humidity data, and 2009 school calendars. The specific measurements the authors considered were age-specific daily counts of positive results for influenza infections from a complete database of over 35,000 specimens submitted for virological testing to the Alberta Provincial Laboratory for Public Health.

The analyses indicated that the ending and recommencement of school terms were temporally associated with the attenuation of one wave of infection and the starting of another. The mathematical models were consistent with school closure reducing influenza transmission among school-age children by more than 50% and that this was a key factor in interrupting community transmission. However, the analyses also suggested that that seasonal changes in weather had a significant effect on the temporal pattern. Two limitations noted in the study were that the data from influenza testing represented only a small sample of all viral infections and the intrinsic simplifying assumptions that mathematical models make in order to make possible simulations and analyses.

The authors conclude that the analyses of data from un-restricted virological testing during this influenza pandemic provided strong evidence that closing schools can have dramatic effects on transmission of pandemic influenza. They argue that school closures seem to be an effective strategy for slowing the spread of pandemic influenza in countries with social contact networks similar to those in Canada.

ECDC Comment (21 February 2012):

The possible role of school closures in mitigating pandemic influenza transmission is a source of ongoing analysis and controversy.(1,2) The initial observational data emerging from the 2009 pandemic is that when the schools closed transmission slumped.(3,4) However as observed by these authors the summer holidays were confounded by good weather and they demonstrate that that weather effects were less powerful than the social distancing. There are many controversies around this topic. One is whether school closures are worth doing at all and another is whether they would always work. The 2009 pandemic was at the mild end of any pandemic spectrum and hence some countries that had envisaged elective school closures as a mitigation strategy rapidly decided they were not worthwhile.(2,6) Generally those were countries that were not used to closing schools as a mitigation policy. Countries that were more used to closing schools during outbreaks of infection proceeded with the interventions.(4) The second controversy is over whether closures would always work in a pandemic. The 2009 pandemic was not only mild it was also one that was quintessentially sensitive to school closures with a low R0 and transmission that was concentrated in younger age groups.(5,6) Pandemics with higher reproductive numbers such as 1918 or infection in all age groups such as 1968 would probably be relatively insensitive to school closure as a mitigation measure.