___ vaccine
(Other definitions for flu that I've seen before include "Minor ailment" , "Winter disease, awfully avian" , "Common disease" , "Viral illness" , "Viral infection (abbr)" .)
(Other definitions for antigen that I've seen before include "Immunity-booster" , "Substance inducing an immune response" , "It stimulates immune responses - tag nine (anag)" , "Something inducing an immune response, neat gin (anag.)" , "toxin" .)