Theoretical and numerical considerations of the assumptions behind triple closures in epidemic models on networks

TL;DR

This paper critically examines the assumptions behind triple closures in pairwise epidemic models on networks, deriving new closures, connecting existing models, and providing guidance on their applicability based on theoretical and numerical analysis.

Contribution

It offers a detailed theoretical analysis of triple closure assumptions, derives new closures, and compares their validity through numerical examples.

Findings

Common closure assumptions can fail with modest degree heterogeneity.

New closures are proposed that better fit certain network structures.

Guidelines are provided for choosing appropriate closures in different scenarios.

Abstract

Networks are widely used to model the contact structure within a population and in the resulting models of disease spread. While networks provide a high degree of realism, the analysis of the exact model is out of reach and even numerical methods fail for modest network size. Hence, mean-field models (e.g. pairwise) focusing on describing the evolution of some summary statistics from the exact model gained a lot of traction over the last few decades. In this paper we revisit the problem of deriving triple closures for pairwise models and we investigate in detail the assumptions behind some of the well-known closures as well as their validity. Using a top-down approach we start at the level of the entire graph and work down to the level of triples and combine this with information around nodes and pairs. We use our approach to derive many of the existing closures and propose new ones and theoretically connect the two well-studied models of multinomial link and Poisson link selection. The theoretical work is backed up by numerical examples to highlight where the commonly used assumptions may fail and provide some recommendations for how to choose the most appropriate closure when using graphs with no or modest degree heterogeneity.