One of the greatest frustrations occurs when a cat develops an infectious disease against which it has been vaccinated. There are five basic reasons for vaccination failure.
1. Ineffective vaccine
The vaccines made by government-licensed manufacturers are potent at the time they leave 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 in maintaining potency. If the vaccine gets too warm during shipment to the distributor or while being stored 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 handled properly during shipment to non-veterinary suppliers. Veterinarians routinely refuse to accept shipments of vaccine if the vaccine is not stored on cold packs at the time of arrival.
It should be noted that the opposite problem can also occur if a vaccine is handled incorrectly. The vaccine can become "virulent", or cause the infection which it is intended to prevent.
2. 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 vaccine is reported to produce immunity in 80-95% of cats that receive it. The vaccines for some of the feline respiratory diseases produce a high level of immunity, but that immunity lasts only eight to ten months in many cats. The vaccine for Feline Infectious Peritonitis is believed to be effective in about 70% of the cats that receive it. Vaccine programs are designed to maximize immunity, however it is important to recognize that no vaccine provides 100% immunity.
3. The cat is too young or is unhealthy when vaccinated
"If the patient has a fever, the immune system will be so preoccupied with the fever that it will respond poorly to the vaccine."
It is essential that the cat have a healthy and functioning immune system in order to respond to the vaccine challenge. If the immune system is immature, such as in a very young animal, or if the patient has a disease that is suppressing the immune system, the vaccine will have little or no effect in stimulating immunity. If the patient 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 prior 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; 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 four months. If her level is low, they may persist 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.
"If a kitten receives a vaccination for rabies before the maternal rabies antibodies are gone, the vaccination is blocked and no immunity develops."
Ideally, a vaccination should be given just after the maternal antibodies are gone, but before the kitten is exposed to infectious organisms. However, there is no practical way to determine just when the maternal antibodies are gone for each of the possible diseases. 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 the vast majority of situations. However, if the maternal antibodies have waned after one vaccination was given and the kitten is exposed to the disease-causing virus or bacterium before the next vaccination occurs, the patient 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 fourteen days from six weeks to sixteen 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.
5. Overwhelming exposure
The vaccine against the feline leukemia virus (FeLV) is effective in about 80-95% of the cats that receive it. However, cats that live with feline leukemia virus infected cats often receive exposure to the virus on a daily basis. Under those conditions, vaccine failures may occur.