Sort of immunity is caught, reportedly (4)
I believe the answer is:
herd
'sort of immunity' is the definition.
(herd immunity)
'caught reportedly' is the wordplay.
'caught' becomes 'heard' (I've seen this before).
'reportedly' indicates a 'sounds like' (homophone) clue.
'heard' sounds like 'HERD'.
'is' acts as a link.
(Other definitions for herd that I've seen before include "Group of, eg, cows" , "People in the mass" , "cattle together" , "Stock of cattle" , "Bunch of cattle" .)