Endemic infectious states below the epidemic threshold and beyond herd immunity
Javier Aguilar, Beatriz Arregui Garc\'ia, Ra\'ul Toral, Sandro Meloni, Jose J. Ramasco

TL;DR
This paper introduces a modified SIR model with external infection inflow to explain persistent low-level epidemic states observed in COVID-19, challenging traditional epidemic threshold concepts.
Contribution
It presents a simple yet effective modification of the SIR model that captures long-living epidemic states and provides analytical insights into their dynamics.
Findings
Model reproduces COVID-19 data features in England.
Identifies epidemic states below and above the traditional threshold.
Challenges existing understanding of epidemic thresholds.
Abstract
In the recent COVID-19 pandemic we assisted at a sequence of epidemic waves intertwined by anomalous fade-outs with periods of low but persistent epidemic prevalence. These long-living epidemic states complicate epidemic control and challenge current modeling approaches as classical epidemic models fail to explain their emergence. Inspired by this phenomenon, we propose a simple mechanism able to reproduce several features observed in real data. Specifically, here we introduce a modification of the Susceptible-Infected-Recovered (SIR) model in a meta-population framework where a small inflow of infected individuals accounts for undetected internal or imported cases. Focusing on a regime where this external seeding is so small that cannot be detected from the analysis of epidemic curves, we find that outbreaks of finite duration percolate in time resulting in overall low but long-living…
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Taxonomy
TopicsCOVID-19 epidemiological studies · Mathematical and Theoretical Epidemiology and Ecology Models · Evolution and Genetic Dynamics
