Resistance providing last of vaccine for pandemic (4)
I believe the answer is:
rife
'pandemic' is the definition.
(thesaurus)
'resistance providing last of vaccine' is the wordplay.
'resistance' becomes 'r' (electronics abbreviation).
'providing' becomes 'if' (I've seen this in other clues).
'last of' indicates one should take the final letters.
The last letter of 'vaccine' is 'e'.
'r'+'if'+'e'='RIFE'
'for' is the link.
(Other definitions for rife that I've seen before include "Widespread sort of fire" , "Endemic" , "Excessively abundant" , "Very common or frequently occurring" , "Of widespread occurrence" .)