The Collaborative International Dictionary of English v.0.48:

immunized \immunized\ adj.
   1. 1 (Med.) rendered less susceptible (to disease) by
      treatment with a vaccine.

   Syn: vaccinated.
        [WordNet 1.5]

   2. rendered insusceptible.
      [PJC]

WordNet (r) 3.0 (2006):

immunized
    adj 1: having been rendered unsusceptible to a disease [syn:
           immunized, immunised, vaccinated]