Information on exported cases identified outside mainland China as at 17 January
On 13 January 2020, Thai health authorities reported an imported case in a person in their 60s who had travelled from Wuhan. This person had onset of an upper respiratory illness on 5 January and landed in Bangkok on 8 January, and was screened out by the thermal scanners at the airport. They did not visit the Southern China Seafood Wholesale Market, but reported visiting another wet market in Wuhan. On 16 January, Japanese health authorities reported a confirmed imported case in a person in their 30s who had travelled from Wuhan and landed in Kanagawa on 6 January. Illness onset was on 3 January and the person has since recovered. The person had not visited any wet market in Wuhan, but had visited a close relative who was in hospital in Wuhan with pneumonia. A third case was reported by Thai health authorities on 17 January. This person was in their 70s and had landed in Bangkok on 13 January, reporting an illness onset on 6 January and unclear history of exposure to the market in Wuhan. On 20 January, the fourth exported case from Wuhan, a person in their 30s visiting South Korea, was identified in Seoul. This person reported no previous visit to wet markets in Wuhan nor contact with any other case within 2 weeks before illness onset. The lack of exposure history to wet markets in Wuhan in two of four generally-mild exported cases indicated that there might be a larger number of undetected infections in Wuhan.
Two family clusters involving five people were reported from 15 to 16 January. One cluster of three close relatives all with illness onset on the same day, were thought to have occurred through a common exposure since they all lived together and worked in the same stall in the Southern China Seafood Wholesale Market. The other cluster involved a couple and may have occurred through human-to-human transmission with a serial interval of 5 days [18].
The basic reproduction number R 0 is defined as the expected number of secondary cases produced by a typical single infection in a completely susceptible population. If there had been only one case infected by human-to-human transmission among the first 41 identified cases by that date, it implies R 0 was 0.02 (i.e. 1/41) [19].