One of the greatest frustrations occurs when a cat develops an infectious disease against which it has been vaccinated. There are five primary reasons for vaccination failure.
The vaccines made by government-licensed manufacturers are potent when they leave the factory, but several things may happen to inactivate them. The most common cause of vaccine inactivation is warming during shipping and handling. Temperature control is critical in maintaining potency. If the vaccine gets too warm during shipment to the distributor or storage at the distributor, it is inactivated. This is a common problem associated with vaccines purchased by mail order, the internet, or from feed stores. The buyer has no way to determine whether the vaccines were correctly handled during shipment to non-veterinary suppliers. Veterinarians routinely refuse to accept vaccine shipments if the vaccines are not stored with cold packs upon arrival.
The opposite problem can occur if a vaccine is mishandled. The vaccine can become virulent (poisonous) or cause the infection it is intended to prevent.
Inherent characteristics of the vaccine
Although most vaccines have a very high success rate in cats, none produces immunity in 100% of cats receiving vaccines. The feline leukemia virus (FeLV) vaccine is reported to produce immunity in 80-95% of cats who receive it. Vaccine programs are designed to maximize immunity; however, it is important to recognize that no vaccine provides 100% immunity.
The cat is too young or is unstable when vaccinated
The cat must have a healthy and functioning immune system to respond to the vaccine challenge. If the immune system is immature, such as in a very young kitten, or the cat has an immune-suppressing disease or is on immuno-suppressive medication, the vaccine will have little or no effect in stimulating immunity. If the cat has a fever, the immune system will be so "preoccupied" with the fever that it will respond poorly to the vaccine. It is important to discuss any health concerns with your veterinarian before vaccinations are given.
Interference due to maternal antibodies
When a kitten is born, she receives immunity-producing proteins, called maternal antibodies, from her mother. Maternal antibodies protect the newborn from the diseases her mother is protected from. Maternal antibodies last only a few weeks in the kitten; their duration is directly proportional to the mother cat’s immunity level. If her immunity level against rabies, for example, is very high, the maternal antibodies for rabies may last up to four months. If her level is low, they may persist for only five or six weeks. As long as these antibodies are present, the kitten is passively protected; however, those antibodies also block the ability to respond to a vaccine challenge. If a kitten receives a vaccination for rabies before the maternal 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, vaccinations should be given shortly after the maternal antibodies are gone but before the kitten is exposed to infectious organisms."
Ideally, vaccinations should be given shortly after the maternal antibodies are gone but before the kitten is exposed to infectious organisms. There is no practical way to determine when the maternal antibodies are gone for each possible disease. Although theoretically, it is possible, it would be costly. Instead, the kitten is given a series of vaccinations at regular intervals, and protection is effective in most situations. However, if the maternal antibodies have waned after one vaccination and the kitten is exposed to the disease-causing virus or bacterium before the next vaccination occurs, the kitten will usually develop the disease.
The solution to this dilemma would be to give more vaccinations in the series. If it is known that the kitten's environment is infected with a particular disease-causing agent, vaccinating every 14 days from six weeks to 16or 20 weeks of age may be recommended. The disadvantage of such a plan is the expense. Instead of giving three vaccinations in the series, six or eight would be given. 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.
The vaccine against the feline leukemia virus (FeLV) is effective in about 80-95% of cats who receive it. However, cats that live with cats infected with the feline leukemia virus often receive exposure to it daily. Under these conditions, vaccine failures may occur.