When initiating vaccination protection, it takes two antigenic exposures to fully alert the immune system of the disease organism. Except for Rabies, a one-time dose. Thus when first protecting a pet from Lymes, Distemper, Parvo, Leptospirosis, Bordetella, Feline Distemper, Leukemia, or Feline Aids the vaccination needs to be given twice to get protection. The optimum time period between the two vaccinations is 3 to 4 weeks. After initial immunization, only one vaccine is required annually.