The Collaborative International Dictionary of English v.0.48:

immunization \immunization\ n.
   the act of making immune (especially by inoculation).

   Syn: immunisation.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

immunization
    n 1: the act of making immune (especially by inoculation) [syn:
         immunization, immunisation]

Moby Thesaurus II by Grady Ward, 1.0:

35 Moby Thesaurus words for "immunization":
   acquired immunity, active immunity, antibody, antigen,
   artificial immunity, congenital immunity, familial immunity,
   immunity, immunity theory, immunization therapy, immunochemistry,
   immunology, immunotherapy, inherent immunity, inherited immunity,
   natural immunity, nonspecific immunity,
   nonsusceptibility to disease, opsonic immunity, passive immunity,
   patch test, phagocytic immunity, racial immunity, resistance,
   scratch test, serotherapeutics, serotherapy, serum therapy,
   side-chain theory, specific immunity, toxin-antitoxin immunity,
   toxin-antitoxin immunization, tuberculin test, vaccine therapy,
   vaccinotherapy