I had something Jan 2020 very similar to what Karl describes as well. I did test positive for flu A at the time, so I'm pretty sure it was H1N1. Caught the coof almost a year later. Whatever hit me Jan 2020 was worse but didn't linger like the coof. Just had a close exposure to someone sick, and natural immunity held up 7 months down the road. When at the doc for the test they said people with prior infections are all over the board with antibodies. Some don't show any after 3-4 months and some people who had it last year are reading as high as the test goes.