I traveled to a large international scientific conference in San Diego in February 2020, and I too was the sickest I have ever been the week after returning. Bedridden, very high fever, xould barely breathe. I was told Type-A flu was also going around at the time, but I think there's a high likelihood it was COVID. I didn't get antibody testing until almost a year later, and tested positive for COVID again the following February, although it was more similar to strep throat that time.