The Cohort Net Reproductive Rate: an empirically grounded approach to R0 for structured populations and epidemics

Christophe F.D. Coste , Steven M. Sait, Mike Fowler, Stephen P Ellner

The net reproductive rate, or per-generation population growth rate R0, is a key concept for population and epidemic dynamics, because persistence versus extinction is often determined by the sign of R0−1. Because it is defined as generation-to-generation growth, it is hard to assess empirically for structured populations or epidemics where generations overlap, and is usually estimated by parameterising dynamic models. Here we introduce the Cohort Net Reproductive Rate R0 (c) – the average per-capita number of offspring or secondary infections produced by individuals born or infected at the same time. We derive a general formula for R0(c) , show that it obeys the same invasion threshold as R0, and analyze when the two coincide or diverge. R0(c) can be estimated from observations on easily identified groups of individuals, is typically easier to calculate, and provides an intuitive meaning for R0 when they coincide.