A. Vaccine Selection
Today, the list of licensed vaccines for just the dog and cat is large and diverse ...approximately 110 canine vaccines and
70 feline vaccines are available. Considerable differences among vaccines for the same antigen exist. The following FAQs center
on scientific issues that should be considered when selecting vaccines for individual patients.
1. Should Modified-Live vaccines be selected over Killed vaccines?
When the option is available-yes. This is particularly true for cats.
Killed (inactivated) vaccines
1. Are less immunogenic than Modified-Live vaccines
2. With the exception of killed rabies vaccine, require at least 2 initial doses in the absence of maternal antibody to immunize,
then at least 7 to 10 additional days before protection is conferred.
3. Contain an adjuvant.
4. Tend to be more commonly associated with acute adverse events (especially pain and acute anaphylaxis) than MLV vaccine.
5. Have been linked to vaccine associated sarcoma (VAS) in cats (especially, killed FeLV and killed Rabies).
6. Cannot revert to virulence (i.e. cause the disease they are intended to prevent)...but, this "safety" issue may be offset by #4 above.
7. Have the shortest duration of immunity.
NOTE: for some diseases, eg, leptospirosis, canine influenza, FIV, canine rabies, the only current option available is a Killed
NOTE: Indications for the use of a Killed vaccine include: 1) vaccination of the pregnant patient (although one has to ask..."why
is it necessary to vaccinate a pregnant dog or cat?") ...and 2) it is recommended that an immune compromised cat (e.g. FeLV
and/or FIV positive) should only be vaccinated with a Killed vaccine. One could apply this rationale to all immune suppressed
patients or patients regardless of the cause (e.g. chemotherapy). However...there are no scientific studies supporting this
Modified-Live (attenuated) vaccines
1. Replicate in the patient post-inoculation and are therefore considered to be more immunogenic than Killed vaccines. A single
dose will immunize in the absence of maternal antibody.
2. May cause transient clinical signs and other potential safety issues (e.g. MLV Canine Distemper vaccine has been implicated
in HOD in susceptible breeds). In puppies, MLV vaccines can produce transient (~5 to 7 days) suppression of cell mediated
immunity that can be clinically significant in certain dogs.
3. Do not contain adjuvant.
4. Are more likely to induce "sterile" immunity than a Killed vaccine.
5. Reversion to virulence of Modified-Live vaccine is technically possible with attenuated vaccine...actual occurrence is
extremely rare with today's vaccines.