It could be that the body is able to fight off the infection better, so they only have mild symptoms. Maybe they don't even know they have it again. The data in

It could be that the body is able to fight off the infection better, so they only have mild symptoms. Maybe they don't even know they have it again. The data in this study was based on health care workers who were tested every 2 to 4 weeks (i.e. not just when they felt really sick).