This study showed that while early infections in Wuhan, China could be linked to the Huanan Seafood Wholesale Market, after January 1, 2020, few infections could – meaning that human-to-human transmission was occurring.
Researchers collected information on the first 425 confirmed cases of COVID-19 in Wuhan, China. Data included demographic characteristics, exposure history, and illness timelines.
The median age was of these patients was 59 years, and 56% were male. Most (55%) cases before January 1, 2020, were linked to the Huanan Seafood Wholesale Market, compared to only 8.6% of cases after that date. The mean incubation period was 5 days, and 95% by 12.5 days. In its early stages, the basic reproductive number was estimated to be 2.2.
Based on these data, while it appeared that most infections were coming from the market, by 2020, cases were clearly being transmitted from human-to-human.