Anecdotal reports that COVID was present in Hubei Province are based on the occurrence of atypical pneumonia during early December 2019. Epidemiologists affiliated with the University of California, San Diego conducted a study on available case data to develop a model to ascertain when the infection first emerged*.  Their technique involved application of a ‘Bayesian molecular clock phylogenetic study’ to ascertain the emergence of the most recent common ancestor of sample strains of SARS-CoV-2.  This approach was combined with forward epidemiological simulations to ascertain how long the virus circulated before it was detected. It was determined that the first case of infection with SARS-COV-2 in Hubei Province occurred between mid-October and mid-November 2019. 

It is postulated that the first virus moving from an animal reservoir to humans was of relatively low infectivity and pathogenicity and was not readily recognized.  They calculated that over 70 percent of SARS-COV-2 zoonotic events would have been self-limiting and would not have resulted in an epidemic.

By December 2019 when the infection was recognized as a novel condition in Wuhan the virus was well established in the population and could have undergone mutation to become more infectious and virulent.

The authors stressed the implications of any delay in recognizing the emergence of any novel virus in relation to emerging zoonotic infections.

* Pekar, J. et al. Timing the SARS-CoV-2 index case in Hubei Province. Science March 18 2021. DOI: 10.1126/science.abf8003.