While this paper[1] doesn't attempt to provide exact dates, the phylogenetic data suggest that there were multiple seeding "events" in each country before outbr

While this paper[1] doesn't attempt to provide exact dates, the phylogenetic data suggest that there were multiple seeding "events" in each country before outbreaks were actually noticeable. [1] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7199730/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7199730/