The FeLV vaccine protects cats against feline leukemia virus, a contagious and potentially fatal infection that suppresses the immune system and can cause cancer. It’s an inactivated vaccine given by injection, typically recommended for kittens and adult cats with any risk of exposure to infected cats. The vaccine works primarily by triggering the body to produce virus-neutralizing antibodies that prevent the virus from establishing a persistent infection in the bloodstream.
How the Vaccine Works
Feline leukemia virus spreads through close contact between cats, and once it takes hold, it can permanently embed itself in a cat’s DNA. The vaccine’s goal is to stop that from happening by priming the immune system before exposure ever occurs. When a vaccinated cat encounters the real virus, its immune system already has antibodies ready to neutralize the virus before it can establish a foothold.
All currently available FeLV vaccines are inactivated, meaning they contain no live virus. Beyond that, they come in different formulations. Some use whole killed virus, others use only specific viral proteins (subunit vaccines), and one type uses a canarypox virus engineered to carry FeLV genes (a recombinant vaccine). The recombinant version is non-adjuvanted, meaning it doesn’t contain the chemical boosters that some other vaccines use to strengthen the immune response. This distinction matters for safety, which we’ll get to below.
How Effective Is It?
Efficacy varies between vaccine types, and the differences can be significant. In a head-to-head challenge study published in Clinical and Vaccine Immunology, an inactivated whole-virus vaccine prevented persistent infection in 100% of vaccinated cats, compared to 91% of unvaccinated controls developing persistent infection. The recombinant canarypox-vectored vaccine had a preventable fraction of 45% in the same study, with 6 out of 10 vaccinated cats still showing viral DNA after challenge.
These numbers come from a deliberately harsh laboratory challenge designed to push vaccines to their limits, so real-world performance may differ. But the takeaway is that not all FeLV vaccines offer the same level of protection, and no vaccine guarantees 100% immunity in every scenario. Vaccination significantly reduces the odds of persistent infection, which is the form of FeLV that causes disease and death.
Which Cats Need It
FeLV spreads through prolonged close contact. Unlike some viruses that travel through the air across a room, feline leukemia virus requires direct cat-to-cat interaction: mutual grooming, sharing food and water bowls, nursing, using the same litter areas, or bite wounds. This makes certain cats far more vulnerable than others.
Cats at highest risk include those that go outdoors (even occasionally), cats living in multi-cat households where not every cat has been tested, cats that might encounter strays, and kittens. Young cats are particularly susceptible to developing persistent infection. Cats that live strictly indoors with no contact with untested cats have the lowest risk, though many veterinarians still recommend vaccinating kittens since their future lifestyle can’t always be predicted.
The American Association of Feline Practitioners considers FeLV vaccination a core vaccine for kittens and a non-core (lifestyle-based) vaccine for adult cats, with the decision hinging on the individual cat’s exposure risk.
Testing Before Vaccination
Your cat should be tested for FeLV before receiving the first vaccine. This is a simple blood test, and it matters for several reasons. If a cat is already infected, the vaccine won’t help. It has no therapeutic value in cats that are already carrying the virus. Vaccinating an infected cat also means exposing that cat to the risk of an adverse reaction with zero benefit.
There’s a practical concern too. If you vaccinate a cat without testing and later discover the cat is FeLV-positive, you won’t know whether the vaccine failed or whether the cat was infected before vaccination ever happened. Testing first gives you a clear baseline.
The Vaccine Schedule
Kittens typically receive two doses. The first is given as early as 8 weeks of age, with a second dose 3 to 4 weeks later. Both doses are needed for full protection. After that initial series, a booster is given one year later. For adult cats continuing to face exposure risk, boosters are generally repeated every two or three years depending on the vaccine type and the veterinarian’s assessment.
For adult cats being vaccinated for the first time, the same two-dose initial series applies, followed by periodic boosters based on ongoing risk.
Common Side Effects
Most cats tolerate the FeLV vaccine well. The most common reaction is mild sluggishness for a day or two after vaccination. Some cats develop a small, firm swelling at the injection site that typically resolves on its own within a few weeks.
Injection-Site Sarcomas
The most serious concern with FeLV vaccination is a rare but aggressive cancer called feline injection-site sarcoma. This tumor develops at or near the spot where a vaccine was injected, sometimes months or even years later. The estimated incidence in the United States is 1 to 4 cases per 10,000 vaccinated cats. In the UK, estimates are lower, roughly 1 case per 5,000 to 12,500 vaccination visits. When these tumors do develop, they metastasize (spread to other parts of the body) in 10% to 28% of cases.
This risk is one reason the vaccine type matters. Adjuvanted vaccines, which contain chemical compounds to boost the immune response, cause significantly more inflammation at the injection site. In a CT imaging study comparing adjuvanted and non-adjuvanted vaccines, swelling reactions were more frequent, more severe, and longer-lasting with the adjuvanted version. On day 7 after injection, the average inflamed volume was nearly 300 times larger in cats that received the adjuvanted vaccine. That inflammation was still visible on imaging at day 84, while cats receiving the non-adjuvanted vaccine showed only mild inflammation at day 7 that resolved by day 21.
Because chronic inflammation at injection sites is believed to play a role in sarcoma development, current guidelines favor non-adjuvanted vaccines when available. Veterinarians also now administer FeLV vaccines in specific locations on the body, typically a hind leg rather than between the shoulder blades, so that if a sarcoma does develop, surgical removal (including amputation if necessary) is more feasible.
Vaccination Is Not a Substitute for Prevention
Even vaccinated cats can potentially become infected, and vaccinated cats that do carry the virus can still spread it to others. The vaccine reduces risk but doesn’t eliminate it entirely. Keeping your cat indoors, testing any new cats before introducing them to your household, and avoiding contact with cats of unknown FeLV status remain the most reliable ways to prevent infection. The vaccine works best as one layer of protection alongside these practical measures.