Feasibility of controlling COVID-19 – Authors' reply
Authors: Rosalind M Eggo, Joel Hellewell, Sebastian Funk
Journal: The Lancet Global Health (2020)

We thank Nian Xiong and Zhicheng Lin for their letter. The on-the-ground experience of medical workers is important for building appropriate models and correctly interpreting their results. In our Article, we defined control as the absence of new infections between weeks 12 and 16 after the start of the outbreak. To achieve control in the model, the outbreak also had to stay at fewer than 5000 cumulative cases. We chose our time points as an approximate range for the length of a short outbreak, and the size of the outbreak for practical reasons, and checked that modelled epidemics that were controlled were never close to reaching 5000 cases. Xiong and Lin certainly raise interesting points about what control could alternatively mean for larger epidemics, but the model was specifically designed to address what control might mean in the context of introductions in new areas. We defined onset as showing symptoms of infection, not viral detection or other approaches that the authors mention. The model simulates isolation on symptom onset among identified contacts of individuals who test positive. We agree that a lack of sensitivity in the tests used to identify cases, as well as the non-specificity of coronavirus disease 2019 (COVID-19) symptoms such as fever, could make achieving control by contact tracing challenging. Although long incubation periods have been reported, studies have estimated that 95% or more of incubation periods are shorter than 12·5 days (in the article Xiong and Lin cite),1Li Q Guan X Wu P et al.Early transmission dynamics in Wuhan, China, of novel coronavirus-infected pneumonia.N Engl J Med. 2020; 382: 1199-1207Crossref PubMed Scopus (10142) Google Scholar shorter than 11·1 days (in the article we used in our model),2Backer JA Klinkenberg D Wallinga J Incubation period of 2019 novel coronavirus (2019-nCoV) infections among travellers from Wuhan, China, 20–28 January 2020.Euro Surveill. 2020; 252000062Crossref Scopus (1005) Google Scholar and shorter than 11·5 days (in a pooled analysis).3Lauer SA Grantz KH Bi Q et al.The incubation period of coronavirus disease 2019 (COVID-19) from publicly reported confirmed cases: estimation and application.Ann Intern Med. 2020; (published online March 10.)DOI:10.7326/M20-0504Crossref PubMed Scopus (3605) Google Scholar The authors mention asymptomatic infection, which we called subclinical and separated from infections that resulted in symptoms in the model. We did not include treated patients who had been discharged as a source of infection—in the model, individuals are isolated on presentation with symptomatic disease and have no further involvement in transmission. If post-discharge transmission occurred, it would likely make control (as defined in the model) difficult to achieve. Our model was not focused on the epidemic trajectory in Wuhan, nor on interventions apart from contact tracing and isolation of cases early in an outbreak. Other models have been used to investigate the rise, control, and subsequent decline of the outbreak in Wuhan and beyond.4Prem K Liu Y Russel TW et al.The effect of control strategies to reduce social mixing on outcomes of the COVID-19 epidemic in Wuhan, China: a modelling study.Lancet Public Health. 2020; (published online March 25.)https://doi.org/10.1016/S2468-2667(20)30073-6Summary Full Text Full Text PDF PubMed Scopus (1303) Google Scholar, 5Kraemer MUG Yang C-H Gutierrez B et al.The effect of human mobility and control measures on the COVID-19 epidemic in China.Science. 2020; (published online March 25.)DOI:10.1126/science.abb4218Crossref Scopus (1670) Google Scholar We declare no competing interests. Feasibility of controlling COVID-19 outbreaks by isolation of cases and contactsIn most scenarios, highly effective contact tracing and case isolation is enough to control a new outbreak of COVID-19 within 3 months. The probability of control decreases with long delays from symptom onset to isolation, fewer cases ascertained by contact tracing, and increasing transmission before symptoms. This model can be modified to reflect updated transmission characteristics and more specific definitions of outbreak control to assess the potential success of local response efforts. Full-Text PDF Open AccessFeasibility of controlling COVID-19We read with interest the modelling study by Joel Hellewell and colleagues,1 on the feasibility of controlling outbreaks of coronavirus disease 2019 (COVID-19) by isolation of cases and contacts. We agree that six main parameters must be considered in predicting the epidemiology—namely, number of initial cases, viral reproduction, isolation delay, tracing probability, asymptomatic transmission (considering a case incubation period of 5·8 days), and subclinical infection. Full-Text PDF Open Access