Presumably they're mostly Americans? That's a big range of time, and having Covid in December 2019 is somewhat unlikely but the chances were not zero (Wuhan is a big city with direct flights to international destinations, including New York and San Francisco), if an American told me they got Covid in April 2020, I would believe them, because the government was keeping that number officially zero through bureaucracy ("You didn't travel to China, we won't test you!" and later "Hey, we've found zero Covid cases!"). There were probably also a number of Americans (or Europeans who soon after travelled to the US) who partied in Ischgl, the Austrian ski resort which became a super-spreader site during the months of February and March.

https://abcnews.go.com/Health/disaster-motion-flights-corona...

And Google Trends for "fever": https://venturebeat.com/2020/09/02/google-releases-data-set-... , but well, it was winter, a comparison of Winter 2019 and 2020 would've been useful.