Final size and convergence rate for an epidemic in heterogeneous population

TL;DR

This paper develops a general SEIR epidemic model for heterogeneous populations, proving the existence, uniqueness, and properties of the final size distribution and the epidemic's convergence behavior.

Contribution

It generalizes previous models by allowing arbitrary contact matrices and traits, proving the final size equation's uniqueness and analyzing the eigenvalue dynamics.

Findings

The final size distribution exists and satisfies the final size equation.

The dominant eigenvalue decreases along trajectories, approaching a limit below 1.

Results hold even with diffusion terms, extending previous finite-trait models.

Abstract

We formulate a general SEIR epidemic model in a heterogenous population characterized by some trait in a discrete or continuous subset of a space R d. The incubation and recovery rates governing the evolution of each homogenous subpopulation depend upon this trait, and no restriction is assumed on the contact matrix that defines the probability for an individual of a given trait to be infected by an individual with another trait. Our goal is to derive and study the final size equation fulfilled by the limit distribution of the population. We show that this limit exists and satisfies the final size equation. The main contribution is to prove the uniqueness of this solution among the distributions smaller than the initial condition. We also establish that the dominant eigenvalue of the next-generation operator (whose initial value is equal to the basic reproduction number) decreases along every trajectory until a limit smaller than 1. The results are shown to remain valid in presence of diffusion term. They generalize previous works corresponding to finite number of traits (including metapopulation models) or to rank 1 contact matrix (modeling e.g. susceptibility or infectivity presenting heterogeneity independently of one another).