Population

We used Sydney’s population in 2015 (12), which was estimated using data from the state of New South Wales (13). The New York population of the same year was derived from the relevant statistical collection (14). We divided both populations into 5-year age groups up through ages 80–84 years and combined the eldest (persons >84 years of age) into a single group (Figure 1, panel A). Each age group was divided into vaccinated and unvaccinated compartments, which were then further subdivided into 3 categories of immunity: immunocompetent, mildly immunosuppressed, and moderate-to-severely immunosuppressed. We assumed that immunosuppressed persons had no residual immunity from vaccination.

Characteristics of population used to model smallpox transmission, by age group, New York, NY, USA, and Sydney, New South Wales, Australia. Characteristics (e.g., size, age, immunosuppression rates) of populations from 2015 were used. A) Total population; B) immunosuppressed population.