# The basic reproduction number, $R_0$, in structured populations

## TL;DR

This paper introduces a simple method to define and derive the basic reproduction number, $R_0$, for Markovian epidemic models in structured populations, applicable to both $SIR$ and $SIS$ models.

## Contribution

It presents a new, straightforward approach to calculating $R_0$ in structured populations, compatible with various epidemic dynamics and models.

## Key findings

- Method applies to $SIR$ and $SIS$ epidemics.
- Expression for $R_0$ matches previous results for household models.
- Approach considers epidemic as a multitype process.

## Abstract

In this paper, we provide a straightforward approach to defining and deriving the key epidemiological quantity, the basic reproduction number, $R_0$, for Markovian epidemics in structured populations. The methodology derived is applicable to, and demonstrated on, both $SIR$ and $SIS$ epidemics and allows for population as well as epidemic dynamics. The approach taken is to consider the epidemic process as a multitype process by identifying and classifying the different types of infectious units along with the infections from, and the transitions between, infectious units. For the household model, we show that our expression for $R_0$ agrees with earlier work despite the alternative nature of the construction of the mean reproductive matrix, and hence, the basic reproduction number.