School closure linked to reduced transmission of pandemic influenza

Earn DJD. Ann Intern Med. 2012;156:173-181.

Issue: March 2012

School closures were associated with reduced transmission of infectious disease and should be considered as a control measure during pandemic influenza, according to study results published in the Annals of Internal Medicine.

To assess correlations between the incidence of pandemic H1N1 (pH1N1) influenza in Alberta, Canada, in 2009 and social-distancing measures, including school closures or weather changes, researchers from the Alberta Provincial Laboratory for Public Health designed mathematical transmission models comparing pH1N1 cases against school calendars and weather patterns.

During the 2009 influenza pandemic, all samples submitted to regional laboratories for respiratory virus testing were referred to the Alberta Provincial Laboratory for Public Health for molecular testing for influenza A, which included validated reverse transcriptase real-time polymerase chain reaction test for influenza A and B. According to the researchers, during the pandemic, specimens were submitted from both community-based health providers and hospitals.

From 2009 to 2010, researchers conducted respiratory virus tests on 35,510 specimens, 19% (n=6,745) of which were identified as positive for pH1N1. According to study results, incidence of pH1N1 declined sharply when schools closed, suggesting that reducing contact between school-age children would reduce transmission rates. The dates on which schools were reopened were followed by the second wave of pH1N1 cases — school-age children exhibited the highest density of confirmed cases, except from late June to late September.
Mathematical models included in the study revealed that typical school closures occurring during the pandemic influenza outbreak reduced transmission among school-age children more than 50% and that seasonal weather changes exerted a significant effect on temporal pattern of the epidemic.

“Our study demonstrates that school-age children were important drivers of pH1N1 transmission in 2009,” researcher David J.D. Earn, PhD, of McMaster’s Michael G. DeGroote Institute for Infectious Disease Research, said in a press release. “The data that we obtained were so good that our plots immediately revealed a huge drop in incidence when schools were closed for the summer. Using state-of-the-art modeling, we then demonstrated that transmission was reduced by at least 50%.”

According to the researchers, Alberta was the only Canadian province to maintain virologic testing throughout the first wave and second wave of the 2009 pandemic, providing researchers with contemporary data to identify the causes of changes in incidence as the pandemic progressed.

“This strongly suggests that closing schools as a preventive measure is a strategy worth seriously thinking about,” Earn said. “The next time a disease like SARS or the 1918 flu emerges, this paper will give policymakers more confidence that closing schools is likely to significantly reduce the rate of transmission.”

Disclosure: The researchers reported financial support from Canadian Institutes of Health Research, Natural Sciences and Engineering Research Council of Canada and the Public Health Agency of Canada.

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