Earlier today, researchers at Imperial College in London estimated that a total of 4,000 cases of Coronavirus in Wuhan City (with an uncertainty range of 1,000 to 9,700) had onset of symptoms by January 18. This compares with a Chinese government report of 440 confirmed cases as of January 21: https://www.imperial.ac.uk/mrc-global-infectious-disease-ana...

According to the Imperial College researchers: "Exit screening (which reportedly came into force on the 15th January) 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."

The researchers also write: "We assume all cases in travelers 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."

If anyone has more recent or higher-quality data or information (especially if it contradicts the Imperial estimates), please post it here!