I am now living in Korea and have seen regularly these circular red shapes on the back of Korean people. Here is a picture. Sorry for the low quality, it is difficult for me to take a picture of someone in a spa, so I had to resort to take a photo from a public Instagram account):
What are these red marks?
I saw them on both men and women, young and old people. They all look healthy at the time of the picture. I could not inspect the marks closely, but they seemed perfectly circular. They were always restricted to the back of the person, and there were always several of them, more or less regualarly organized.
My best guess is a vaccination mark.
Note: as I don't really have any idea of what it is, I have difficulties to find relevant tags. Any help is welcome.