I've been in Japan from mid-Oct until mid-Dec in 2019 and in late October I got sick like hell. People were talking about the flu but it was not just that I can tell. For me the symptoms were way worse than before when having the flu, and only later realized that every single symptom I had matched with Covid. At least the first two waves spared me at home. But one of my friends was not so successful. He was in Japan too until the end of the summer, and back home he had to go to the hospital with a nasty bilateral pneumonia. The doctors were confused back then, but finally he recovered. Months later it turned out that it's one of the most severe complications of Covid.

Have read some paper stating that the first infections have happened in May, but couldn't find it.