Feasibility of controlling COVID-19 outbreaks by isolation of cases and contacts

Hellewell, Joel; Abbott, Sam; Gimma, Amy; Bosse, Nikos I; Jarvis, Christopher I; Russell, Timothy W; Munday, James D; Kucharski, Adam J; Edmunds, W John; Funk, Sebastian; Eggo, Rosalind M

To achieve control of 90% of outbreaks, 80% of contacts needed to be traced and isolated for scenarios with a reproduction number of 2·5 ( figure 3 ). The probability of control was higher at all levels of contact tracing when the reproduction number was 1·5, and fell rapidly for a reproduction number of 3·5. At a reproduction number of 1·5, the effect of isolation was coupled with the chance of stochastic extinction resulting from overdispersion, 19 which is why some outbreaks were controlled even at 0% contacts traced. Isolation and contact tracing decreased transmission, as shown by a decrease in the effective reproduction number ( figure 3) . When the basic reproduction number was 1·5, the median estimate rapidly fell below 1, which indicated that control was probable. For the higher transmission scenarios, a higher level of contact tracing was needed to bring the median effective reproduction number below 1. The effect of isolation without contact tracing can be seen at 0%, where the effective reproduction number was lower than the simulated basic reproduction number because of rapid isolation (and ceasing transmission) of cases.