Strange attractors in a dynamical system inspired by a seasonally forced SIR model

TL;DR

This paper demonstrates that a seasonally forced SIR model can exhibit strange attractors and chaos even when the basic reproduction number suggests disease elimination, highlighting complex dynamics induced by seasonality.

Contribution

It provides a rigorous proof of strange attractors in a biological model with seasonality, extending understanding of chaos in epidemiological systems.

Findings

Existence of persistent strange attractors when R0<1

Numerical evidence of chaos in seasonally forced models

Backwards bifurcation prevents disease elimination under certain conditions

Abstract

We analyze a multiparameter periodically-forced dynamical system inspired in the SIR endemic model. We show that the condition on the \emph{basic reproduction number} $\mathcal{R}_0 < 1$ is not sufficient to guarantee the elimination of \emph{Infectious} individuals due to a \emph{backward bifurcation}. Using the theory of rank-one attractors, for an open subset in the space of parameters where $\mathcal{R}_0<1$, the flow exhibits \emph{persistent strange attractors}. These sets are not confined to a tubular neighbourhood in the phase space, are numerically observable and shadow the ghost of a two-dimensional invariant torus. Although numerical experiments have already suggested that periodically-forced biological models may exhibit observable chaos, a rigorous proof was not given before. Our results agree well with the empirical belief that intense seasonality induces chaos. This work provides a preliminary investigation of the interplay between seasonality, deterministic dynamics and the prevalence of strange attractors in a nonlinear forced system inspired by biology.