Thread 1/10: On January 17 we released estimates of the scale of the nCoV-19 outbreak in China based on an analysis of the number of cases detected outside mainland China.

Thread 2/10: Since then, cumulative confirmed cases reported by the Chinese authorities have increased 10-fold, to 440 by January 22nd.The number of detected outside China with symptom onset by 18th January had increased to 7 in the same time.

Thread 3/10: Here we report updated estimates of the scale of the epidemic in Wuhan, based on an analysis of flight and population data from that city. Our estimate of the number of cases in Wuhan with symptoms onset by January 18th is now 4,000.

Thread 4/10: The uncertainty range is 1,000-9,700 reflecting the many continuing unknowns involved in deriving these estimates. Our central estimate of 4,000 is more than double our past estimates, a result of the increase of the number of cases detected outside China from 3 to 7

Thread 5/10: Our estimates should not be interpreted as implying the outbreak has doubled in size in the period 12th January to 18th January

Thread 6/10: 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.

Thread 7/10: Our analysis suggests that the nCoV-19 outbreak has caused substantially more cases of moderate or severe respiratory illness in Wuhan than have currently been detected.

Thread 8/10: However, recent rapid increases in officially reported confirmed case numbers in China suggest that case detection and reporting has been substantially enhanced in recent days.

Thread 9/10: With further refinements and expansion of surveillance (for instance, to primary care providers) it is to be hoped that the differences between our estimates and official case numbers will lessen further.

Thread 10/10: Given the increasing evidence for human-to-human transmission, enhancing rapid case detection will be essential if the outbreak is to be controlled.