Report 2: Estimating the potential total number of novel Coronavirus cases in Wuhan City, China

Abstract

We estimate that a total of 4,000 cases of 2019-nCoV in Wuhan City (uncertainty range: 1,000 – 9,700) had onset of symptoms by 18th January 2020 (the last reported onset date of any case) [15]. Our estimates should not be interpreted as implying the outbreak has doubled in size in the period 12th January to 18th January – delays in confirming and reporting exported cases and incomplete information about dates of symptom onset together with the still very small numbers of exported cases mean we are unable to estimate the epidemic growth rate at the current time. This estimate is based on the following assumptions: • Wuhan International Airport has a catchment population of 19 million individuals [1]. • There is a mean 10-day delay between infection and detection, comprising a 5-6 day incubation period [16,17] and a 4-5 day delay from symptom onset to detection/hospitalisation of a case (the cases detected in Thailand and Japan were hospitalised 3 and 7 days after onset, respectively) [4,18]. • Total volume of international travel from Wuhan over the last two months has been 3,301 passengers per day. This estimate is derived from the 3,418 foreign passengers per day in the top 20 country destinations based on 2018 IATA data [19], and uses 2016 IATA data held by Imperial College London to correct for the travel surge at Chinese New Year present in the latter data (which has not happened yet this year) and for travel to countries outside the top 20 destination list. • Exit screening (which reportedly came into force on the 15th January [13]) had no impact on exported cases reported up to 16th January. Exit screening may have reduced exports in recent days, in which case our baseline prediction may be an underestimate of the true number of cases in Wuhan. • We assume all cases in travellers flying to destinations outside mainland China are being detected at those destinations. This may well not be the case. If cases are being missed in other countries, our baseline prediction will underestimate the true number of cases in Wuhan. • We now report uncertainty as the range spanned by the 95% confidence intervals of the first three scenarios in Table 1. Thus, our uncertainty range represents uncertainty in key assumptions as well as statistical assumptions.