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Hong Kong Study Finds that Closing Schools May Not Have Helped Slow the Spread of Flu

By Jennifer B. Nuzzo, S.M., September 26, 2008

A new study published in Emerging Infectious Diseases (EID) suggests that Hong Kong’s efforts to close schools for two weeks during the 2008 influenza season may have done little to limit the community-wide spread of influenza.1

On March 12, 2008, the government of Hong Kong made international headlines when it announced that it would close all primary schools, kindergartens, special schools, and day nurseries following the influenza-related deaths of 3 children. Schools were then closed the next day, one week before the annual week-long Easter break.

In previous influenza epidemics elsewhere in the world, the impact of school closings on illness rates has been mixed. A study from Israel reported a decrease in indicators of respiratory infections after a 2-week teacher strike.2 On the other hand, the WHO has noted that when schools closed for a winter holiday during the 1918 influenza pandemic in Chicago, “more influenza cases developed among pupils . . . than when schools were in session.”3

To examine the efficacy of school closure in Hong Kong, researchers examined influenza and influenza-like illness (ILI) activity data from before, during, and after the school closure (from December 1, 2007 through April 26, 2008). A review of clinical specimens, outpatient visits for influenza-like illness, and influenza-related hospital admissions during this period indicates that morbidity during the 2008 influenza season had peaked and was waning by the time authorities decided to close schools. Similarly, the percent of children absent from sentinel childcare centers and primary schools also peaked prior to the school closures and “returned to low levels after the closures.”1

The investigators also used laboratory isolate and outpatient data to generate daily epidemic curves that modeled transmission dynamics of influenza in the community. This simulation found no significant reduction in rates of transmission during the period in which schools were closed, which suggests that “the effect of the intervention was not substantial.”

Although reports of pediatric deaths motivated Hong Kong officials to close schools, the authors found no evidence that the 2008 flu season was any more severe than in previous years. Despite evidence that the “2007-2008 strains of influenza virus in Hong Kong…were not well matched to the trivalent inactivated vaccine specified for the season,” both outpatient consultations and hospital admissions during the 2008 influenza season were comparable to the preceding 9 and 2 flu seasons, respectively.

The results suggest that closing schools for two weeks “did not have a substantial effect on community transmission” of influenza. It is possible that the ultimate decline in influenza cases seen after the school break may have occurred naturally without any intervention. However, the authors note that “given the limitations on an uncontrolled natural experiment on the population level” such results should “be interpreted with caution.” They note that the routine surveillance data may not accurately represent the true incidence of influenza in communities. For one, influenza-like illness rates may be skewed by the presence/absence of other co-circulating respiratory viruses. Similarly, the rate of influenza viruses isolated during and after the school closure may be a biased estimate of true viral incidence if increased concern and awareness following announcement of the school closure caused an artificial increase in testing. The authors also note that it may be difficult to make direct inferences from the epidemic curves generated in the study, “because changes in the epidemic curve may lag behind changes in the underlying transmission dynamics,” as had been shown for SARS.

The authors acknowledge that it is impossible to determine from these results whether “influenza might have continued to circulate for a longer period had the school closures not been implemented.” To that end, they stress that if future “public health decisions are to be made on the basis of prospective surveillance, these systems must be improved to reflect real-time or near real-time reporting and analysis.”

Although this study does not provide conclusive evidence regarding the efficacy of school closures in limiting community-wide transmission of influenza, it does raise serious questions about the utility of this measure. In recent months, ethicists have called attention to potential adverse consequences of school closure policies and the relative lack of consideration of these issues in U.S. pandemic plans.4 Noting the lack of data to support school closures, they urge policy makers to consider school closures with caution, stressing the importance of first gathering more evidence about the efficacy of this measure. Given that school closure is a cornerstone of U.S. pandemic planning efforts, such questions deserve a harder look.


  1. Cowling BJ, Lau EHY, Lam CLH, Cheng CKY, Kovar J, Chan KH, et al. Effects of school closures, 2008 winter influenza season, Hong Kong. Emerging Infectious Diseases Online Edition. October 2008. Accessed September 25, 2008.

  2. Heymann A, Chodick G, Reichman B, Kokia E, Laufer J. Influence of school closure on the incidence of viral respiratory diseases among children and on health care utilization. Pediatr Infect Dis J. 2004;23:675–7.

  3. World Health Organization Writing Group. Nonpharmaceutical public health interventions for pandemic influenza, national and community measures. Emerg Infect Dis. 2006;12:88–94.

  4. Berkman BE. Mitigating pandemic influenza: the ethics of implementing a school closure policy. J Public Health Manag Pract. 2008 Jul-Aug;14(4):372-8.