TOKYO—The Japanese government Tuesday was quick to deny a U.S. warning for Americans to avoid traveling to Japan would have an impact on Olympians wanting to compete in the postponed Tokyo Games. U.S. officials cited a surge in coronavirus cases in Japan caused by virus variants that may even be risky to vaccinated people. They didn’t ban Americans from visiting Japan, but the warnings could affect insurance rates and whether Olympic athletes and other participants decide to join the games that open on July 23. Most metro areas in Japan are under a state of emergency and expected to remain so through mid-June because of rising serious COVID-19 cases that are putting pressure on the country’s medical care systems. That raises concern about how the country could cope with the arrival of tens of thousands of Olympic participants if its hospitals remain stressed and little of its population is vaccinated. …