The Dec 2019 bug also hit me (living in NYC). I wasn't mentally myself for three weeks. I'm not even sure I fully recuperated. It was intense, and worse than when I eventually did experience COVID a couple years later.

From what I read at the time, there was a non-COVID flu at this time throughout the east coast. Tests months later did not show I had COVID antibodies. It remains to me a remarkable sickness unlike anything else I've experienced.

Same with my sister and her husband (London) Worst flu they ever had, I was looking after their kids. That said, later they tested positive for Covid antibodies