Seasonal synchronization of influenza in the United States older adult population

Influenza is a significant health problem in the United States leading to serious illness, hospitalization, and death among the older adult population. Current influenza surveillance systems do not address annual geographic differences in the predictability and periodicity of influenza and little previous research has been conducted to examine the geographic variability of influenza in the older adult population. The objectives of our research were to examine national and state level trends of traveling waves as well as the relationship between peak timing and intensity of influenza in the US older adult population. A total of 248,889 hospitalization records were extracted from Centers for Medicare and Medicaid Service for the influenza seasons 1991-2004. Annual Harmonic Regression models were used to quantify the peak timing and absolute intensity for each of the 48 contiguous states and Washington, DC. We found that individual influenza seasons show synchrony with consistent late or early timing occurring across all 48 states during each influenza season in comparison to the average across the 13 influenza seasons. Peak timing and absolute intensity were inversely related (rs=-0.5) where the earlier the peak timing the more influenza in a given population. Our findings suggest that it is imperative that influenza surveillance should systematically report peak timing and intensity on a state level for each influenza season so that predictions can be made for future influenza seasons leading to prevention of severe disease in the vulnerable, older adult population.