Contributors: Adam Kucharski, Tim Russell, Charlie Diamond, Yang Liu, CMMID nCoV working group, John Edmunds, Sebastian Funk, Rosalind Eggo.
Note: this is preliminary analysis and has not yet been peer-reviewed.
To understand how human-to-human transmission varied in Wuhan during the early stages of the 2019-2020 nCoV outbreak and project forward based on current trends.
• To estimate the early dynamics of transmission in Wuhan, we fitted a mathematical model to multiple available datasets on international exported cases from Wuhan and cases in Wuhan. Fitting to multiple data sources rather than a single dataset (or data point) is particularly useful for estimates in real-time, because some datasets may be unreliable.
• Transmission was a random process in the model, and could vary over time – this means the model can uncover fluctuations in transmission during the early stages of the outbreak. Our group previously used a similar analysis to understand the dynamics of Ebola in Liberia.
• We assumed that the chance of cases being exported from Wuhan to other countries depended on the number of cases in Wuhan, the number of outbound travellers (accounting for travel restrictions after 23rd January), and the relative connectivity of different countries. We considered the 30 countries outside China most at risk of exported cases in the analysis. The model accounts for delays in symptom onset and reporting (see methods below).
• We estimated that the median effective basic reproduction number, Rt, had likely been fluctuating between 1.6-3.1 prior to travel restrictions being introduced on 23rd Jan (Figure 1F). (The effective reproduction number is the average number of secondary cases generated by a typical infectious individual at a given point in time, assuming a fully susceptible population).
• If Rt continues to vary as it has in Wuhan, we projected that the outbreak would peak in mid-to-late-February (Figure 1E). There is substantial uncertainty about what the exact height and timing of the peak might be. However, our projections aligned reasonably well with observed confirmed cases so far in Wuhan in early February (Figure 2F) - as we get more data in the coming days, we will be able to refine these projections.
• Based on the median reproduction number observed during January before travel restrictions were introduced, we estimated that a single introduction of 2019-nCoV with SARS-like or MERS-like individual-level variation in transmission would have a 20–30% probability of causing a large outbreak (Figure 2A). Assuming SARS-like variation and Wuhan-like transmission, we estimated that once more than three infections have been introduced into a new location, there is an over 50% chance that an outbreak will occur (Figure 2B).