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

Outbreak control was defined as no new infections between 12 and 16 weeks after the initial cases. Outbreaks that reached 5000 cumulative cases were assumed to be too large to control within 12-16 weeks, and were categorised as uncontrolled outbreaks. Based on this definition, we reported the probability that an outbreak of a severe acute respiratory syndrome coronavirus 2-like pathogen would be controlled within 12 weeks for each scenario, assuming that the basic reproduction number remained constant and no other interventions were implemented. The probability that an outbreak is controlled gives a one-dimensional understanding of the difficulty of achieving control, because the model placed no constraints on the number of cases and contacts that could be traced and isolated. In reality, the feasibility of contact tracing and isolation is likely to be determined both by the probability of achieving control, and the resources needed to trace and isolate infected cases. 32 We therefore reported the weekly maximum number of cases undergoing contact tracing and isolation for each scenario that resulted in outbreak control. New cases require their contacts to be traced, and if these numbers are high, it can overwhelm the contact-tracing system and affect the quality of the contact-tracing effort. 33 It is likely that the upper limit on contacts to trace varies from country to country.