An elaboration of theory about preventing outbreaks in homogeneous populations to include heterogeneity or preferential mixing

The goal of many vaccination programs is to attain the population immunity above which pathogens introduced by infectious people (e.g., travelers from endemic areas) will not cause outbreaks. Using a simple meta-population model, we demonstrate that, if sub-populations either differ in characteristics affecting their basic reproduction numbers or if their members mix preferentially, weighted average sub-population immunities cannot be compared with the proportionally-mixing homogeneous population-immunity threshold, as public health practitioners are wont to do. Then we review the effect of heterogeneity in average per capita contact rates on the basic meta-population reproduction number. To the extent that population density affects contacts, for example, rates might differ in urban and rural sub-populations. Other differences among sub-populations in characteristics affecting their basic reproduction numbers would contribute similarly. In agreement with more recent results, we show that heterogeneous preferential mixing among sub-populations increases the basic meta-population reproduction number more than homogeneous preferential mixing does. Next we refine earlier results on the effects of heterogeneity in sub-population immunities and preferential mixing on the effective meta-population reproduction number. Finally, we propose the vector of partial derivatives of this reproduction number with respect to the sub-population immunities as a fundamentally new tool for targeting vaccination efforts. •Population-immunity thresholds are useful only in homogeneous, proportionally-mixing populations.•Meta-population effective reproduction numbers, ℜv, and related quantities always are useful.•Heterogeneity in variables affecting sub-population reproduction numbers is relevant.•Together with preferential mixing among sub-populations, such heterogeneity increases ℜv.•The partial derivatives of ℜv with respect to sub-population immunities indicates the optimal strategy.