Feasibility of controlling 2019-nCoV outbreaks by isolation of cases and contacts

Abstract

Background:To assess the viability of isolation and contact tracing to control onwards transmission from imported cases of 2019-nCoV. Methods:We developed a stochastic transmission model, parameterised to the 2019-nCoV outbreak. We used the model to quantify the potential effectiveness of contact tracing and isolation of cases at controlling a 2019 nCoV-like pathogen. We considered scenarios that varied in: the number of initial cases; the basic reproduction number R0; the delay from symptom onset to isolation; the probability contacts were traced; the proportion of transmission that occurred before symptom onset, and the proportion of subclinical infections. We assumed isolation prevented all further transmission in the model. Outbreakswere deemed controlled if transmission ended within 12 weeks or before 5000 cases in total.We measured the success of controlling outbreaks using isolation and contact tracing, and quantified the weekly maximum number of cases traced to measure feasibility of public health effort.Findings: While simulated outbreaks starting with only 5 initial cases, R0 of 1.5 and little transmission before symptom onset could be controlled even with low contact tracing probability, the prospects of controlling an outbreak dramatically dropped with the number of initial cases, with higher R0, and with more transmission before symptom onset. Across different initial numbers of cases, the majority of scenarios with an R0 of 1.5 were controllable with under 50% of contacts successfully traced. For R0 of 2.5 and 3.5, more than 70% and 90% of contacts respectively had to be traced to control the majority of outbreaks.The delay between symptom onset and isolation played the largest role in determining whether an outbreak was controllable for lower values of R0. For higher values of R0 and a large initial number of cases, contact tracing and isolation was only potentially feasible when less than 1% of transmission occurred before symptom onset.Interpretation: We found that in most scenarios contact tracing and case isolation alone is unlikely to control a new outbreak of 2019-nCov within three months. The probability of control decreases with longer 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.

Early dynamics of transmission and control of COVID-19: a mathematical modelling study

“…This factor highlights the importance of rapid case identification and subsequent isolation and other control measures to reduce the chance of onward chains of transmission. 20 Our analysis highlights the value of combining multiple data sources in analysis of COVID-19. For example, the rapid growth of confirmed cases globally during late January, 2020, with case totals in some instances apparently doubling every day or so, would have had the effect of inflating R t estimates to implausibly large values if only these recent datapoints were used in our analysis.…”

Beyond R0: Heterogeneity in secondary infections and probabilistic epidemic forecasting

“…Critically, this large range stems from uncertainty in the heterogeneity of secondary infections. If the heterogeneity is large, sustained transmission is mostly maintained by so-called "super-spreading" events, then the outbreak is both more likely to end stochastically, less likely to spread extensively, as well as easier to manage with contact tracing, screening and infection control [24]. With less heterogeneity, the outbreak almost certainly cannot be contained and we must prepare for a pandemic of 2019-nCoV [54,55].…”

“…The range of potential R0 comes from a 95% confidence interval using a early data and a classic deterministic models [18,19]. The range of dispersion parameter k comes from analogy with severe acute respiratory syndrome [13,24]. Most importantly, with fixed average, the dispersion parameter is inversely proportional to the variance of the underlying distribution of second cases.…”

“…Despite numerous estimates of 2019-nCoV's R 0 (ranging from 1.5 to just under 4 [18][19][20][21][22][23]), as we discussed above in the comparison of Ebola virus disease and 1918 influenza, whether-like SARS-2019-nCoV can be contained or-like 2009 H1N1-2019-nCoV will cause a pandemic depends largely on the heterogeneity in the number of secondary infections. Indeed, recent work modeling the effectiveness of contact tracing and isolation on preventing 2019-nCoV concluded that the probability of containing the outbreak was significantly lower if 2019-nCoV heterogeneity in secondary infections was low, like influenza, as compared to higher, like SARS [24]. To more fully quantify how heterogeneity in the number of secondary infections affects outbreak size, we turn towards network epidemiology and derive an equation for the total number of infected individuals using all moments of the distribution of secondary infections.…”

Estimating number of global importations of COVID-19 from Wuhan, risk of transmission outside mainland China and COVID-19 introduction index between countries outside mainland China

“…The impact of control measures after initial detected cases is not included in our model, but may alter the risk of onward transmission and should be considered in further iterations of the model as in other modelling work considering impact interventions under different transmission scenarios. 20,21 Other countries that we estimate to be at high risk of local transmission, but that have not yet detected transmission 18 include Indonesia, Cambodia, Canada and the Philippines. For all these places, our results suggest there could be a consideration of expanding testing of pneumonia or influenza like illness cases beyond those who have currently travelled, to find community transmission as soon as possible after it occurs, as has been ongoing in Singapore 19 and has been recommended recently in some areas of the US 22 .…”