Winter holidays delay seasonal influenza epidemic peaks and shift disease risk toward adults because of changes in contact patterns. These findings may inform targeted influenza information and vaccination campaigns during holiday periods.

Background: The seasonality of influenza is thought to vary depending on changes in environmental factors, human contact, and travel patterns. During winter holidays, when children are out of school, typical patterns of potential disease-causing contact and travel change. The aim of this research is to understand the impact of winter school holidays on age-specific and spatial flu transmission, and the resulting size and spread of the epidemic. We hypothesize that reduced contacts among children during school holidays will lower their infection rates, infection rates among adults will subsequently decrease as fewer children are sick, and increased travel during the holidays will hasten the spread of flu across the country.Methods: Using aggregated medical claims data from physician offices and hospitals in the United States, we examined influenza dynamics before, during, and after the winter holidays from 2001 to 2009. We quantified the changes to transmission and characterized holiday-associated changes to epidemic trajectories among children and adults in a spatial context. Informed by this empirical evidence, we used a deterministic, age-specific spatial metapopulation model to compare models with and without holiday-associated behavioral changes to understand the effects of holidays on flu season trajectories and spatial spread. Results:In analyzing eight seasons of medical claims data, we observed a mean 15% drop in the effective reproductive number immediately after Christmas. Rates of influenza-like illness among children experienced a sharp dip and recovery during and after the holidays, while adults experienced more gradual dips and recoveries during these periods. The model results demonstrated that both child and adult flu incidence dipped during the holiday period, but trends among adults lagged those in children. The peaks of the child and adult flu epidemics were delayed when the holiday was introduced, but the attack rate did not change significantly for either age group. In the medical claims data and the model-generated data with the holiday, the spatial synchrony of epidemic trajectories before and after the holiday remained similar. In the model-generated data without the holiday, however, epidemic trajectories became spatially asynchronous.Conclusions: Our work suggests that winter holidays delay the epidemic peaks and increase the spatial synchrony of natural trajectories for seasonal influenza in the United States every year. These findings have implications for the design of influenza intervention strategies, such as the proper timing and duration of school closures, and the spatial and demographic reallocation of vaccines.