[roywiggins]

Because winter is flu season, and most of the time the symptoms are impossible to distinguish, even when you're looking?

Sure, China has way more public health capacity than it used to, but we know that COVID can spread silently in a community for a month without anyone noticing, even when we are looking. It happened in California and Seattle in January 2020. Why wouldn't that have happened in, say, rural China in October?

[ekianjo]

There were apparently already people with COVID symptoms in Italy back in December 2019. That said, China was already aware of the virus in late 2019. It's all well known.

[roywiggins]

Well, that's what I mean. Nobody knew COVID was circulating in Italy at the time either. It's easy to miss a new respiratory virus. For it to originate one place and by chance end up exploding in a different metropolitan area doesn't seem unlikely at all.

[Pyramus]

In fact, simply from a modelling perspective, this is very likely scenario.

If you take an unknown diseases with an R of 2-3, what you will see is a number of smaller clusters, some dying off, before you get the one cluster that becomes the pandemic.