It's not unusual for players to retire. Fifteen may be an extreme number but we need to also take into consideration the tennis season. The Miami open is the last tournament on the hard court before everything switches to the clay. There were most likely Clay specialists just using the Miami open as a warm up but with no intention of winning and every intention of dropping out. I used to trade tennis matches on the betting exchanges and got caught on numerous occasions with retirements, some of which were very dubious. Until the WTA / ATP do something, this will most likely continue. Don't get me wrong, I'm not suggesting they were all bogus retirements, but I am saying that it's highly unlikely that they were all vaccine related also!