Influence of Population Immunosuppression and Past Vaccination on Smallpox Reemergence

The authors developed a SEIR (susceptible, exposed, infected, recovered) smallpox transmission model for New York City and Sydney, Australia that accounted for age-specific population immunosuppression and residual vaccine immunity. They found that residual immunity was less influential on disease rates in Sydney, which has a vaccination coverage rate half that of New York, and that age-specific death rates were lower in older age groups with higher residual immunity in both cities. Infection and death rates increased in both cities when immunosuppressed parameters were included in the model.