Spatial models for pandemic influenza: Extending human movement models across international borders

Modeling the spread of pandemic influenza can help public health officials decide when and where to concentrate prevention, detection, and intervention efforts. As influenza is transmitted from person to person, models for the spread of influenza and influenza-like viruses require an understanding of human movement and travel patterns. A recent movement model, known as the radiation model, has been shown to accurately predict the movement of people within the contiguous United States. Using commuter data from the 2000 US Census, we identify geographic regions of poor model fit, and demonstrate that this radiation model formulation does not accurately predict cross-border movement to Canada and Mexico. We propose a modified radiation model that takes the borders into account, adjusting the probability that a worker commutes to a foreign country. Our modifications to the radiation model significantly improve its ability to predict international movement from the United States to Canada and Mexico. The modifications particularly improve the fit of the model to commuter data for US counties near international borders. The modified radiation model could be applied to simulate the spread of pandemic influenza in North America.