Invisible spread of SARS-CoV-2 – Authors' reply
Authors: Adam J Kucharski, Rosalind M Eggo
Journal: The Lancet Infectious Diseases (2020)

We thank Nian Xiong and colleagues for their response to our Article.1Kucharski AJ Russell TW Diamond C et al.Early dynamics of transmission and control of COVID-19: a mathematical modelling study.Lancet Infect Dis. 2020; (published online March 11.)https://doi.org/10.1016/S1473-3099(20)30144-4Summary Full Text Full Text PDF PubMed Scopus (1635) Google Scholar Although we separated individuals into exposed and infectious compartments in the basic model, we also considered a sensitivity analysis whereby people became infectious in the second half of their incubation period, and obtained the same conclusion (Article appendix p 12). We allowed the reproduction number, R, to vary over time in our model, rather than simply fix this value, to capture possible variation in transmission as a result of control measures and behaviour change. However, our median estimate for the reproduction number in mid-January of 2·4 is consistent with other estimates from the same period by use of a fixed R.2Li 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 (10666) Google Scholar As there is a delay from infection to symptom onset to hospitalisation, our model incorporated a delay to account for the time it takes for changes in transmission to be reflected in the observed data. Our estimate for transmission reduction was similar to that in another study, which focused on case counts in Wuhan and estimated that R had declined to around 1·3 by the last week of January, 2020.3Lauer 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 (3773) Google Scholar We disagree that our assumed incubation period was inappropriate; our assumption of a 5·2 day (SD 3·7) value is consistent with later studies that have estimated a similar value.4Wang C Liu L Hao X et al.Evolving epidemiology and impact of non-pharmaceutical interventions on the outbreak of coronavirus disease 2019 in Wuhan, China.medRxiv. 2020; (published online March 6.)DOI:10.1101/2020.03.03.20030593Google Scholar Xiong and colleagues raise an important point about the need to disentangle the precise drivers of the reduction in transmission. Although our model estimated an overall reduction in transmission, we did not have sufficient data to identify precisely how social distancing, quarantine, travel restrictions, and other lifestyle changes influenced these dynamics. There is no clear evidence that variation in the viral genome has driven changes in transmission,5MacLean OA Orton R Singer JB et al.Response to "On the origin and continuing evolution of SARS-CoV-2".http://virological.org/t/response-to-on-the-origin-and-continuing-evolution-of-sars-cov-2/418Date: 2020Date accessed: March 27, 2020Google Scholar but sequence data could be useful for understanding the expansion and size of the outbreak. In a follow-up study, we have considered the effect of the timing and length of a lockdown, and the implications for ongoing transmission.6Prem K Liu Y Russell 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 (1366) Google Scholar As more data become available on the timing of control measures and subsequent outbreak dynamics, we agree that it will be crucial to evaluate the effectiveness of measures to provide a robust evidence base for future policy- making. We declare no competing interests. Early dynamics of transmission and control of COVID-19: a mathematical modelling studyOur results show that COVID-19 transmission probably declined in Wuhan during late January, 2020, coinciding with the introduction of travel control measures. As more cases arrive in international locations with similar transmission potential to Wuhan before these control measures, it is likely many chains of transmission will fail to establish initially, but might lead to new outbreaks eventually. Full-Text PDF Open AccessInvisible spread of SARS-CoV-2We read with interest Adam Kucharski and colleagues' mathematical modelling study of the early dynamics of coronavirus disease 2019 (COVID-19).1 We agree that a stochastic transmission model might best fit with the reality around the Huanan Seafood Wholesale Market, which was the origin of the COVID-19 outbreak2 and 1 mile away from our hospitals in Wuhan. We appreciate the work making use of different datasets and considering travel. However, we have concerns about the clinical and strategic values of this work. Full-Text PDF