All Kupot Cholims in general or just the one by Mir?
Most young Chutznikim in the Mir have Meuchedet.
I have no data where exactly they took the blood for the antibody test for each of the peoples tests, but it's not like there's a clinic across the street that gives blood tests. (There's a doctor, no nurse services).
The bachurim probably took it either on Shivtei Yisrael or Malachi/Chagi, the marrieds where ever they live; Romema, Arzei, RE, Har Nof, Bayit Vegan, RBS, Kiryat Sefer
The Israeli talmidim have whatever kupa they have.