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Vaccination Failure in Cats

It is a very frustrating and difficult situation when a cat develops an infectious disease against which it has been vaccinated. Although most owners consider that administration of a vaccine confers complete immunity, it is still possible for disease to be contracted in selected situations. Fortunately, this is relatively uncommon. When vaccination failure does occur, it is usually due to one of the following:

1. INEFFECTIVE VACCINE

   Vaccines produced by USDA-licensed manufacturers are rigorously evaluated for safety and potency prior to shipment from the factory; however, several things may happen to inactivate them. The most common cause of vaccine inactivation is warming of the vaccine during shipping and handling. Temperature control is critical to maintaining potency. If the vaccine gets too warm during shipment to the distributor or while being stored at the distributor, it is inactivated. Also, some vaccines can be damaged by exposure to ultraviolet light or high-intensity visible light. This is prevented by storage in the dark. These are typical of problems associated with vaccines purchased by through mail order or from feed stores. The buyer has no way to determine whether the vaccines were handled properly during shipment to non-veterinary suppliers. Veterinarians routinely refuse to accept shipments of vaccine if the vaccine is improperly packaged at the time of delivery.

2. INHERENT CHARACTERISTICS OF THE VACCINE

   Although most of our vaccines have a very high success rate in cats, none produce immunity in 100% of cats receiving vaccines. The feline leukemia virus vaccine produces immunity in only 80-90% of cats that receive it. The vaccine for some of the feline respiratory viruses produces a high level of immunity, but that immunity lasts only 8-10 months in some cats.

3. THE CAT IS TOO YOUNG OR IS UNHEALTHY

   It is essential that the cat have a functional immune system in order to respond to the vaccine challenge. If the immune system is very immature, such as a very young animal, or if the patient has a disease which is suppressing the immune system, the vaccine will have little or no effect in stimulating immunity. For example, if the patient has a fever, the immune system will be so “occupied” with the fever that it will respond poorly to the vaccine. Concurrent administration of certain drugs and particular types of stress may also interfere with the cat’s ability to respond to vaccination.

4. INTERFERENCE DUE TO MATERNAL ANTIBODIES

   When a kitten is born, it receives immunity-producing proteins from its mother. These are called maternal antibodies. Maternal antibodies protect the newborn from the same diseases against which the mother was protected. Maternal antibodies last only a few weeks in the kitten and their duration is directly proportional to the level of immunity in the mother cat. If her immunity level against rabies, for example, is very high, the maternal antibodies for rabies may last up to 4 months. If her level is low, they may persist only 5 or 6 weeks. As long as these antibodies are present, the kitten is protected; however, those antibodies also block a vaccine challenge. If a kitten receives a vaccination for rabies before the rabies antibodies are gone, the vaccination is blocked and no immunity develops. The same holds true for the other components of the vaccines - the temporary immunity received from the mother can interfere with all of the vaccinations.

   Ideally, a vaccination should be given just after the maternal antibodies are gone but before the kitten is exposed to infectious agents. However, it is not practical to determine just when the maternal antibodies are gone for each of the possible diseases. It can be done, but the expense would be prohibitive. Instead, the kitten is given a schedule for vaccination at regular intervals. The timing of this plan is successful in the vast majority of situations. However, if the maternal antibodies are gone and the kitten is exposed to the disease-causing agent before the next vaccination occurs, the patient can develop the disease.

   The solution to this dilemma would be to give more vaccinations in the series. If the premises are known to be infected with a particular disease-causing agent, we may recommend vaccinating every 10 to 14 days from age 6 weeks to 16 weeks. The disadvantage of such a plan is the expense. Instead of giving 3 vaccinations in the series, we would be giving 6 or 8. This would result in more than double the cost of the routine vaccine series. The potential benefits and risks of extra vaccinations can be discussed with your veterinarian.

5. VACCINE DELIVERY

   Some vaccines are intended to be given by only the intramuscular or subcutaneous routes. Also, some vaccines must be reconstituted prior to administration. Finally, the appropriate dose of vaccine must be given. If the manufacturer’s recommendations are not precisely followed, the intended response may not be achieved. For this reason, it is usually best to allow a veterinary professional to administer vaccines.