Quasi-Neutral theory of epidemic outbreaks

TL;DR

This paper introduces a quasi-neutral model for epidemic outbreaks that explains scale-invariance and power-law distributions without fine-tuning, drawing parallels to models in population genetics and ecology.

Contribution

It demonstrates that the theory of accidental pathogens is a quasi-neutral model with critical behavior similar to the voter model, simplifying understanding of epidemic scale invariance.

Findings

Outbreak sizes follow a power-law distribution.

The model exhibits critical behavior akin to the voter model.

Explains scale invariance in epidemics through neutral theory.

Abstract

Some epidemics have been empirically observed to exhibit outbreaks of all possible sizes, i.e., to be scalefree or scale-invariant. Different explanations for this finding have been put forward; among them there is a model for "accidental pathogens" which leads to power-law distributed outbreaks without apparent need of parameter fine tuning. This model has been claimed to be related to self-organized criticality, and its critical properties have been conjectured to be related to directed percolation. Instead, we show that this is a (quasi) neutral model, analogous to those used in Population Genetics and Ecology, with the same critical behavior as the voter-model, i.e. the theory of accidental pathogens is a (quasi)-neutral theory. This analogy allows us to explain all the system phenomenology, including generic scale invariance and the associated scaling exponents, in a parsimonious and simple way.