June 6, 2011

Flu Pandemic Study Supports Social Distancing

Photo of empty swings outside an elementary school

A new study found that mandatory school closures and other social distancing measures reduced influenza transmission rates in Mexico during the 2009 pandemic. The results suggest that similar measures could be useful for fighting future influenza pandemics.

Pandemic flu comes rarely — only 3 times in the 20th century — but can be devastating. It’s caused by a sudden major shift in circulating flu viruses, such as when a virus jumps from an animal to humans. Most people have no immunity to such new viruses. The 2009 H1N1 flu was first diagnosed in Mexico in April 2009. With components from flu viruses known to affect pigs, birds and humans, it rapidly spread across the world to become pandemic.

Social distancing measures can be implemented during unusual infectious diseases outbreaks. These interventions include closing schools, movie theaters and restaurants, and cancelling 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 closures on a local basis during the 2009 pandemic, but the impact of these interventions has yet to be evaluated.

A research team led by Dr. Gerardo Chowell of NIH’s Fogarty International Center and Arizona State University, Tempe, set out to evaluate the impact of nonmedical factors on the pandemic’s spread. They used influenza surveillance data covering 40% of the population that was compiled by a large private health system, the Mexican Institute for Social Security. Overall, about 23% of the people tested were positive for the virus. The analysis appeared on May 24, 2011, in PLoS Medicine.

The researchers identified a 3-wave pandemic profile for 2009. The first wave occurred in spring in the Mexico City area. A second wave came in summer in the southeastern region, and a third wave came in the fall. The onset of this third pandemic wave occurred within 2 to 5 weeks of the beginning of the fall school term and was marked by increased disease incidence among school-age children.

The very young were hit the hardest during the pandemic, not the elderly who are typically at high risk for influenza. Relatively few cases were reported among seniors during the pandemic period. However, seniors suffered the highest risk of developing severe disease.

Significantly, the 18-day period of mandatory school closures and other social distancing measures were associated with a 29% to 37% reduction in influenza transmission rates.

These findings highlight variation in pandemic influenza incidence and severity among age groups. They also reveal the importance of school cycles on the transmission dynamics of this disease. The results suggest that school closure and other measures could help to mitigate future influenza pandemics.

“We believe this study has implications for improving preparedness plans in future pandemics,” says Chowell. In a previous influenza pandemic in the 19th century, the majority of deaths occurred 2 years after the initial wave. Chowell emphasizes,“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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