Before revision:  The challenge is that selective isolation of the sick, an often viable and effective strategy, is insufficient against COVID-19 , due to its relatively long incubation period, in which exposed individuals experience no symptoms, but still contribute to the spread.
After revision:  The challenge is that selective isolation of the sick, an often viable and effective strategy, is insufficient against COVID-19 , due to its relatively long incubation period, in which exposed individuals experience no symptoms, but may already contribute to the spread.