Tokyo — Japan’s Covid-19 case numbers have plummeted to the lowest in nearly a year just as other parts of Asia are struggling with surging infections, leaving health experts perplexed and raising concern of a winter rebound. New daily cases in Tokyo dropped to 87 on Monday, the lowest tally since November 2, and from more than 5,000 a day in an August wave that hammered the capital’s medical infrastructure. The pattern is the same across the country. After a slow start, Japan has made rapid progress in its vaccination campaign and almost six months of emergency distancing restrictions have...