Mathematical modeling has affected vaccination policy throughout the developed world and, via the WHO, elsewhere. Policy goals vary with disease and setting, but preventing outbreaks is common. This is attained by exceeding the population immunity threshold. While immunity seems to be at or above the threshold for many vaccine-preventable diseases in the US, policymakers are concerned about heterogeneity due to personal-belief exemptions to vaccination. We use a mathematical model to evaluate the impact of such heterogeneity on our efforts to prevent disease outbreaks. We have also used a model which includes seasonally forced transmission rates to predict 2009 pandemic H1N1 influenza in the US.