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Holidays reduce influenza transmission, delay trajectory of seasonal epidemics

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January 3, 2017

The holiday season not only reduces influenza transmission in the United States, it delays the trajectory of seasonal influenza epidemics, researchers said.

These effects are related to school closings, according to Shweta Bansal, PhD, assistant professor of biology at Georgetown University, and colleagues.

“In comparing the effects of two purported mechanisms driving holiday dynamics — school closure and increased travel — we were surprised to find that changes due to school closure explained nearly all of the delay in peak timing and increase in spatial synchrony in our model,” Bansal and colleagues wrote in the Journal of Infectious Diseases. “Travel restrictions have been shown to produce little to no effect on the spread of pandemic influenza, and our study adds to this literature on the minimal effect of holiday-related travel re-routing on spatial influenza transmission.

“While holiday travel is more commonly linked with seeding and synchronizing flu in multiple locations, we found that school closures, and more specifically, reductions in the average number of holiday contacts and not the changes in mixing among age groups, could in fact create a dampening and synchronizing spatial effect. Thus our study is consistent with other model-based studies supporting the idea that school closures may be successful when timed early during the epidemic.”

Bansal and colleagues used a health records database that captures approximately 60% of all U.S. outpatient visits to obtain weekly visits for influenza-like illness (ILI) during eight influenza seasons from October 2001 to May 2009. They defined the epidemic period as October through March and established 2-week “before,” “during” and “after” periods around the holiday season, with Christmas marking the beginning of the “during” period.

In all eight influenza seasons, Bansal and colleagues observed a temporary reduction in influenza activity during the weeks following Christmas. Transmission decreased by approximately 15% in most seasons, falling below the epidemic threshold after Christmas, then rebounding to epidemic levels within a few weeks. Seasonal peaks occurred an average of 5 weeks after the holidays.

Bansal and colleagues also examined weekly ILI medical claims, which showed the risk for disease shifted toward adults during and after the holidays.

“We posit that these patterns may be driven by altered interaction patterns due to children being home from school or families (rather than business travelers) traveling for the holidays,” they wrote.

When they simulated influenza epidemics with and without the impact of school closures and travel, Bansal and colleagues found that models including school closures and the holiday period delayed the peak of influenza season, while traveling did not have the same effect. Total epidemic sizes were not affected, however.

“We found that holiday-associated changes to age-specific contact patterns reproduced many of the patterns observed in our empirical data,” Bansal and colleagues wrote. “In the model, holiday contact patterns were responsible for causing temporary reductions in flu activity during the holiday period, shifting the risk of disease from children to adults, pushing the epidemic peak later in time, and increasing the synchrony in flu incidence during the holiday period.”  by Gerard Gallagher

Disclosures: The researchers report no relevant financial disclosures.

itj+ Perspective

Amesh A. Adalja

The newly released study by Bansal and colleagues that demonstrates the holiday season delays the peak of influenza has many important implications. As schools are closed during this period, children have a diminished opportunity to spread influenza among each other — a major driver of seasonal influenza — and instead they infect adults. This changed contact pattern, which pushes the peak of influenza season later, affords the opportunity for those not vaccinated against influenza to still protect themselves via vaccination for the rest of the influenza season which is yet to peak. Influenza vaccination, limited as it is in protection, is the chief means we currently have at hand to prevent influenza and this study provides evidence that even during the holiday season, it is not too late to benefit from vaccination.

Amesh A. Adalja, MD, FACP

Infectious disease physician at the University of Pittsburgh; Spokesman for the Infectious Diseases Society of America

Disclosure: Adalja reports no relevant financial disclosures.