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

Our model did not include other control measures that might decrease the reproduction number and therefore also increase the probability of achieving control of an outbreak. At the same time, it assumed that isolation of cases and contacts is completely effective, and that all symptomatic cases are eventually reported. Relaxing these assumptions would decrease the probability that control is achieved. We also make the assumption that contact is required for transmission between two individuals, but transmission via fomites might be possible. This type of transmission would make effective contact tracing challenging, and good respiratory and hand hygiene would be crucial to reduce this route of transmission, coupled with environmental decontamination in health-care settings. For this reason, we used contact-tracing percentage intervals of 20% to avoid indicating more precision in the corresponding probability of control than the model could support. We simplified our model to determine the effect of contact tracing and isolation on the control of outbreaks under different scenarios of transmission; however, as more data becomes available, the model can be updated or tailored to particular public health contexts. The robustness of control measures is likely to be affected both by differences in transmission between countries, but also by the concurrent number of cases that require contact tracing in each scenario. Practically, there is likely to be an upper bound on the number of cases that can be traced, which might vary by country, and case isolation is likely to be imperfect. 34 We reported the maximum number of weekly cases during controlled outbreaks, but the capacity of response efforts might vary. In addition to the number of contacts, other factors could decrease the percentage of contacts that can be traced, such as cooperation of the community with the public health response.