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

We explored a range of scenarios informed by the latest evidence on transmission of COVID-19. Similar analyses using branching models have already been used to analyse the Wuhan outbreak to find plausible ranges for the initial exposure event size and the basic reproduction number. 15, 18 Our analysis expands on this work by including infectiousness before the onset of symptoms, case isolation, explicit modelling of case incubation periods, and time to infectiousness. A key area of uncertainty is whether, and for how long, individuals are infectious before symptom onset, and whether subclinical infection occurs; both are likely to make the outbreak harder to control. 22 Whether, and how much, transmission occurs before symptoms is difficult to quantify. Under-reporting of prodromal symptoms, such as fatigue and mild fever, is possible; thus, transmission might not truly be occurring before symptoms, but before noticeable symptoms. There is evidence of transmission before onset, 30 and so we used 15%. Increased awareness of prodromal symptoms, and therefore short delays until isolation-as seen in the SARS outbreak in Beijing in 2003 35 -would increase control of outbreaks in our model. If contact tracing included testing of non-symptomatic contacts, those contacts could be quarantined without symptoms, which would decrease transmission in the model. Costs associated with additional testing might not be possible in all contexts.