Factors that make an infectious disease outbreak controllable

TLDR

The study examines how the timing of infectiousness relative to symptom onset affects the effectiveness of isolation and contact tracing, using a mathematical model. The study aims to identify properties of emerging infectious agents that determine the success of isolating symptomatic individuals and tracing/quarantining contacts. The authors use a mathematical model to estimate the timing of infectiousness relative to symptom onset and the proportion of asymptomatic transmission for SARS‑CoV, HIV, smallpox, and pandemic influenza, drawing on published data. The analysis shows that success of isolation and contact tracing depends largely on the proportion of transmission before symptom onset and on R0, with SARS and smallpox being more controllable, and recommends prioritizing estimation of asymptomatic/presymptomatic transmission during novel outbreaks.

Abstract

The aim of this study is to identify general properties of emerging infectious agents that determine the likely success of two simple public health measures in controlling outbreaks, namely ( i ) isolating symptomatic individuals and ( ii ) tracing and quarantining their contacts. Because these measures depend on the recognition of specific disease symptoms, we investigate the relative timing of infectiousness and the appearance of symptoms by using a mathematical model. We show that the success of these control measures is determined as much by the proportion of transmission occurring prior to the onset of overt clinical symptoms (or via asymptomatic infection) as the inherent transmissibility of the etiological agent (measured by the reproductive number R 0 ). From published studies, we estimate these quantities for two moderately transmissible viruses, severe acute respiratory syndrome coronavirus and HIV, and for two highly transmissible viruses, smallpox and pandemic influenza. We conclude that severe acute respiratory syndrome and smallpox are easier to control using these simple public health measures. Direct estimation of the proportion of asymptomatic and presymptomatic infections is achievable by contact tracing and should be a priority during an outbreak of a novel infectious agent.