Vaccination Failures in Cats

Why would a vaccine fail?

Vaccines are designed to protect against disease, so it is frustrating when that doesn't happen and a cat develops a disease against which it has been vaccinated. Vaccine failures are rare, but they can occur. There are five primary reasons why a vaccine may not work.

1. Ineffective vaccine

Vaccines made by government-licensed manufacturers are potent (active) when they leave the factory but may lose potency on their way to the veterinarian. Most commonly, warming during shipping and handling can inactivate a vaccine.

Temperature control is critical in maintaining potency. This problem is often associated with vaccines purchased directly by pet owners via mail order, internet, or feed stores, because the buyer does not know whether the vaccines were correctly handled during shipment. Veterinarians do not accept vaccine shipments unless the vaccines are stored with cold packs.

2. Inherent characteristics of the vaccine

Most vaccines have a very high success rate in cats, but none produces immunity in 100% of vaccinated cats. The feline leukemia virus (FeLV) vaccine is reported to produce immunity in 80% to 95% of cats that receive it. Vaccine programs are designed to maximize immunity, but it is important to recognize that no vaccine provides 100% immunity.

3. The cat is not healthy when vaccinated

The cat’s immune system must function properly to respond appropriately to a vaccine. If the immune system is immature or is suppressed by disease or medication, it will not respond. If the cat has a fever, the immune system may respond poorly to the vaccine because it is already responding to the fever. It is important to discuss any health concerns with your veterinarian before your cat is vaccinated.

4. Interference of maternal antibodies

When kittens are born, they receive immunity-producing proteins, called maternal antibodies, from their mother. Maternal antibodies protect the newborn from diseases the mother is protected against. Maternal antibodies stay protective in the kitten only for a few weeks. Maternal antibodies protect a kitten against infection, but that also includes blocking the effects of a vaccine. This prevents a vaccinated kitten from building their own protection against the disease until the maternal antibody number decreases.

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 exactly when maternal antibodies disappear for each possible disease. 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, the kitten will likely develop the disease.

5. Overwhelming exposure

Cats and kittens can be exposed to large amounts of viruses or bacteria, especially in overcrowded or unsanitary conditions. In these circumstances, vaccine failures may occur. In particular, cats that live with FeLV-infected cats are often exposed to that virus daily, which may lead to vaccine failures.