The authors used a mathematical model to assess if isolation and contact tracing are able to control onwards transmission from imported cases of COVID-19. They developed a stochastic transmission model to quantify the potential effectiveness of contact tracing and isolation of cases. They used scenarios that varied in the number of initial cases, basic reproduction number (R0), delay from symptom onset to isolation, probability that contacts were traced, proportion of transmission that occurred before symptom onset, and proportion of subclinical infections. It was assumed that isolation prevented all further transmission. Outbreaks were deemed controlled if transmission ended within 12 weeks or before 5000 total cases. Simulated outbreaks starting with five initial cases, an R0 of 1·5 and 0% transmission before symptom onset could be controlled even with low contact tracing probability. However, the probability of controlling an outbreak decreased as the number of initial cases increased or when R0 was 2·5 or 3·5, or with more transmission before symptom onset. Across different initial numbers of cases, the majority of scenarios with an R0 of 1·5 could be controlled with <50% of contacts successfully traced. To control the majority of outbreaks, for R0 of 2·5 >70% of contacts had to be traced, and for an R0 of 3·5 >90% of contacts. The delay between symptom onset and isolation was found to have the largest role in determining whether an outbreak could be controlled when R0 was 1·5. For R0 values of 2·5 or 3·5, if there were 40 initial cases, contact tracing and isolation were only potentially feasible when <1% of transmission occurred before symptom onset. Therefore in most scenarios, highly effective contact tracing and case isolation is enough to control a new outbreak of COVID-19 within 3 months.
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