  The World Health Organisation (WHO) has very strongly recommended testing and
isolation as a strategy for controlling the ongoing COVID-19 pandemic. The goal
of this paper is to quantify the effects of detection and isolation in formal
models of epidemics of varying complexity. A key parameter of such models is
the basic reproduction ratio. We show that an effective detection and isolation
strategy leads to a reduction of the basic reproduction ratio and can even lead
to this ratio becoming lower than one.

Populations and Evolution