Snowbirds and influenza: Spatiotemporal dynamics of influenza in the Medicare population by age, month, state of residence, and provider state

Understanding the geographic and temporal dynamics of influenza is an essential part of public health preparation and prevention of this deadly disease, especially for vulnerable older adults, the population group most detrimentally affected by influenza. Older adults are more likely than other population groups to migrate seasonally within the United States. The purpose of this analysis is to describe the spatiotemporal distributions of influenza-associated hospitalizations highlighting discrepancies between Medicare beneficiaries' state of residence and provider state. We abstracted approximately 12 million Medicare hospitalizations from influenza and pneumonia (ICD-9CM 480-487) and tabulated the results by provider state, state of residence, age, and month of admission for 1991-2006. The largest proportion of discrepancies between provider state and state of residence occurred in December (13.4%) and January (13.7%). These discrepancies occurred primarily for those whose provider state was Florida, but whose state of residence was a Northeastern state, including New York (17.0%) and Massachusetts (15.5%). There were also comparatively large discrepancies in June (8.7%) and July (9.1%), but for these months the state of residence was Florida and the provider state tended to be in the Northeast. There was also a significant annual increase in the proportion of the Medicare population treated in Florida from December to February, but whose residence was a Northeastern state (p<0.006) from 1991 to 2006. These discrepancies were highest for individuals aged 69 to 77. Understanding influenza dynamics may reflect larger demographic dynamics and may help to ensure that adequate care is provided to seniors to treat influenza.