Proactive social distancing mitigates COVID-19 outbreaks within a month across 58 mainland China cities

Abstract

In early 2020, cities across China enacted strict social distancing measures to contain emerging coronavirus (COVID-19) outbreaks. We estimated the speed with which these measures contained community transmission in each of 58 Chinese cities. On average, containment was achieved 7.83 days (SD 6.79 days) after the implementation of social distancing interventions, with an average reduction in the reproduction number (Rt) of 54.3% (SD 17.6%) over that time period. A single day delay in the implementation of social distancing led to a 2.41 (95% CI: 0.97, 3.86) day delay in containment. Swift social distancing interventions may thus achieve rapid containment of newly emerging COVID-19 outbreaks.

Figure 1.. COVID-19 introductions, transmission and containment in Mainland China cities prior to February 15, 2020.

We demonstrate the methodology for two provincial capitals: (a, c) Xi’an and (b, d) Nanjing. (a,b) The estimated daily incidence of imported and locally-transmitted COVID-19 cases and the implementation times for local social distancing measures. (c,d) The estimated daily time-varying reproduction numbers (R_t), with the green line and shading indicating the median and 95% credibility bounds for R_t respectively. We calculate the number of days from the first reported imported case until the upper 95% CI of R_t drops below one (χ) for 58 cities in Mainland China. (e) The distribution of χ across 58 cities in Mainland China (also see Appendix Figure S2). The average duration of outbreaks is 21 days (95% CI: 7.95, 35.1). Based on an AIC comparison between Gamma, Lognormal and Weibull distributions fitted via maximum likelihood to the data (Appendix Table S3), we find that the χ are roughly Weibull distributed with scale 22.94 (95% CI: 21.12, 24.91) and shape 3.28 (95% CI: 2.68, 4.02), as indicated by black line, (f) The distribution of time between the first locally reported case and the first social distancing measure resembles a Weibull distribution with scale 14.24 (95% CI: 13.01,15.60) and shape 2.98 (95% CI: 2.44,3.65).