Feline Immunodeficiency Virus (FIV) is a global retrovirus that affects domestic cats, often compared to human HIV due to the way it compromises the immune system. The virus slowly destroys white blood cells, specifically T-lymphocytes, leaving the cat susceptible to secondary infections and certain cancers. FIV is primarily transmitted through deep, penetrating bite wounds that allow virus-laden saliva to enter the bloodstream. FIV is species-specific and poses no risk of transmission to humans or other non-feline pets. The vaccine introduces inactivated viral components to the cat’s immune system, stimulating the production of protective antibodies. This immune response is intended to prevent or reduce the severity of infection if the cat is later exposed to the live virus.
Vaccine Efficacy and Candidate Selection
The FIV vaccine is generally categorized as a “non-core” vaccine, meaning it is recommended only for cats with a demonstrable risk of exposure. This classification is partly due to the vaccine’s variable efficacy against the diverse strains of the virus found worldwide. The commercially available inactivated whole-virus vaccine contains components from two specific FIV subtypes, A and D, but at least five different subtypes (A through E) circulate globally. While the vaccine offers significant protection against the strains it contains, its ability to protect against other, “heterologous” subtypes, such as the widely prevalent subtype B, is less consistent. This variability is a consideration for veterinarians, especially in regions where non-vaccine strains are common. The vaccine is typically reserved for high-risk individuals, such as cats with outdoor access who frequently fight, or those in multi-cat households where the FIV status of all residents is unknown. Conversely, a cat living strictly indoors, without contact with potentially infected outdoor cats, is considered low-risk and generally does not require the vaccine. The decision to vaccinate is an individualized assessment that balances a cat’s lifestyle risk against the vaccine’s limitations.
Impact on FIV Diagnostic Testing
The most significant complication of the FIV vaccine is its interference with standard diagnostic testing. The most common screening tests for FIV, such as the ELISA or rapid point-of-care tests, detect FIV-specific antibodies in the cat’s blood. These antibodies are typically produced only when a cat has been naturally infected. Since the vaccine is designed to stimulate the immune system to produce these same antibodies, a vaccinated cat will test positive on these screening tests. This makes it impossible to distinguish between a vaccinated cat and a truly infected cat, resulting in a “false positive” for infection status. This diagnostic ambiguity creates serious challenges, particularly in situations involving animal shelters or the adoption process, where a cat’s full medical history may be unknown. A positive antibody test in a vaccinated cat can lead to an incorrect label of “FIV-positive.” To definitively determine the true infection status of a vaccinated cat, a more complex and expensive test, such as a Polymerase Chain Reaction (PCR) assay, is required. The PCR test detects the actual genetic material of the virus, rather than just the antibodies, providing a reliable result for a cat with a positive antibody screen.
Vaccination Protocol and Schedule
The initial vaccination series for the FIV vaccine requires multiple doses to establish sufficient immunity. For kittens or adult cats receiving the vaccine for the first time, a series of three injections is typically administered. Each primary dose is spaced two to four weeks apart, allowing the immune system time to build its initial protective response. Full protection is achieved two to three weeks following the final dose in the primary series. Periodic booster vaccinations are necessary to maintain immunity, with the manufacturer recommending an annual single dose. The specific timing of booster shots may be adjusted by the veterinarian based on the cat’s individual risk factors and local disease prevalence.
Safety Profile and Adverse Effects
The FIV vaccine, like any injection, carries a risk of adverse effects, although most are mild and temporary. Common post-vaccination reactions include non-specific systemic signs such as a slight fever, reduced appetite, and lethargy, which usually resolve on their own within 24 to 48 hours. Localized reactions at the injection site, such as transient pain or a small, firm swelling under the skin, can also occur. A more serious, though rare, risk associated with certain injectable vaccines is the development of feline injection-site sarcomas (FISS). These are aggressive, cancerous tumors that can develop weeks, months, or even years after the injection. While the incidence rate is low, estimated to be between one and four cases per 10,000 vaccinated cats, the severity of the tumor is why some veterinarians recommend the FIV vaccine only for high-risk patients. Because of this rare but serious complication, current veterinary guidelines recommend that all vaccines be administered in specific, easily monitored locations, such as the lower distal limbs. This strategic placement is intended to facilitate aggressive surgical treatment, such as limb amputation, if a sarcoma were to develop, which offers the best chance for a successful outcome. Owners should monitor any swelling at the injection site; if a lump persists for more than three weeks, or increases in size, veterinary attention is required.