News Release

Tuesday, May 24, 2011

Mexican flu pandemic study supports social distancing

Fogarty research published in PLoS Medicine.

Eighteen-day periods of mandatory school closures and other social distancing measures were associated with a 29 to 37 percent reduction in influenza transmission rates in Mexico during the 2009 pandemic. The research was carried out by scientists at the Fogarty International Center at the National Institutes of Health and published in PLoS Medicine.

The social distancing measures implemented by the Mexican health authorities in spring 2009 were effective in reducing disease transmission by more than one-third, the study found. Social distancing interventions can be implemented during unusual infectious diseases outbreaks and include school closing, closure of movie theaters and restaurants, and the cancellation of large public gatherings. Mexico implemented a nationwide mandatory school closure policy during an 18-day period in late April and early May 2011. The United States implemented school closure interventions on a local basis during the 2009 pandemic, but the impact of these interventions has yet to be evaluated.

The study was led by Gerardo Chowell, Ph.D., a Fogarty investigator and faculty member at Arizona State University, Tempe. His team provides the first comprehensive epidemiological description of the age, geographical and severity patterns of the 2009 pandemic in Mexico.

The authors applied mathematical modeling to influenza surveillance data compiled by a large private health system, the Mexican Institute for Social Security, which covers 40 percent of the population.

A three-wave pandemic profile was identified throughout Mexico. The initial wave occurred in spring 2009 in the Mexico City area. A second wave was noted in summer 2009 in the southeastern region. Finally, a third wave occurred in fall 2009. The onset of the third pandemic wave in fall 2009 occurred within two to five weeks of the beginning of the fall school term, coinciding with increased disease incidence among school-age children.

The hardest hit were the very young, not the elderly who are typically at high risk with influenza. There were a few cases reported among seniors during the pandemic period, but the most severe influenza-related infections were seen in those 5 to 14 years old, as well as in infants.

Overall, the researchers’ findings highlight variation in pandemic influenza incidence and severity among age groups, and reveal the importance of school cycles on the transmission dynamics of this disease. The importance of school—children for pandemic influenza transmission is based on the findings that school closure effectively reduced influenza transmission in spring 2009 in Mexico, together with a large increase in influenza cases among school children coinciding with schools reopening.

The study suggests that school closure and other measures could be useful to mitigate future influenza pandemics.

"We believe this study has implications for improving preparedness plans in future pandemics," said Chowell. Noting that, in a previous influenza pandemic in the 19th century, the majority of deaths occurred two years after the initial wave, he emphasized that "we must remain vigilant and continue to monitor the circulation and health burden of the pandemic A/H1N1 and co-circulating influenza viruses in the coming years."

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