I had the same symptoms that I had the second time. The second time was confirmed by PCR-test.

But, of course, it could have been an equivalent disease with the same symptoms: fever, mental-fog, sore-throat.

It is just the timing, the fact that Osaka is visited by many Chinese nationals and the feeling was so similar that makes me quite sure that it was covid-19.

Those are symptoms of literally every airway infection known to man. Is is much, much, much more likely you had a flu/heavy cold instead of having Covid 1 month before the first public Chinese cases and 3 months before first European cases

Also that if the first disease wasn't covid, getting covid the second time around isn't that unlikely, but if the first disease was covid, getting covid again was shockingly unlikely (not impossible) because of the degree of immunity granted by the first infection.