Feline Leukemia Virus (FeLV) Vaccine - Cats

Why It’s Important:

Feline leukemia virus is often called a “friendly cat” disease because it can spread during normal, non-aggressive contact (unlike FIV which spreads mainly by bites). Roughly 50% of cats who persistently carry FeLV die within 2–3 years of diagnosis due to related diseases (such as cancers or severe infections)​. Young cats are especially susceptible. By vaccinating, you are protecting your cat from a lifelong infection that would significantly shorten their life and quality of life. FeLV causes immunosuppression – infected cats get frequent illnesses because their bodies can’t fight germs well. It can also trigger leukemia or lymphoma tumors. The FeLV vaccine is considered a core vaccine for all kittens up to one year old​, because young cats are at highest risk and benefit greatly from early protection. For adult cats, it’s considered non-core (optional) and is recommended based on the cat’s lifestyle: if they go outdoors, live with FeLV-positive cats, or may be exposed to other cats of unknown status​. Even indoor cats sometimes escape or have direct contact with the occasional foster or new kitten in the home, so many vets advise at least the initial kitten series. Importantly, FeLV is not contagious to people or dogs, only among cats. But within the cat population, it’s a serious and prevalent threat that vaccination has helped dramatically reduce over past decades​.

What to Expect: The FeLV vaccine is an injection, often given in a limb (commonly a hind leg, as per recommendations to manage rare vaccine-site reactions). Kittens usually receive this vaccine starting around 8 weeks of age, then again 3–4 weeks later as a booster​. This initial series (two doses) is essential to build strong immunity​. After that, if your cat remains at risk, boosters are given annually (or in some cases every 2 years depending on the vaccine brand and vet’s discretion). At a routine vet visit, the process is quick – the vet may first perform a simple FeLV blood test (a small prick on the leg to get a few drops of blood) to ensure your cat is currently FeLV-negative before vaccinating​. This is important: if a cat already has FeLV, vaccinating won’t help and isn’t indicated. The test is often a combo snap test along with FIV, with results in about 10 minutes during the appointment. If negative, the vet gives the vaccine under the skin. Some cats may feel a bit of the needle pinch but many don’t react. Try to keep the visit low-stress (bringing a favorite blanket or treats can help). After the shot, monitor your cat for any soreness or lethargy, just as you would after other vaccines. If your kitten is getting multiple vaccines in one visit (like FeLV plus rabies, etc.), the vet might give them in different sites and could even split them into separate visits to minimize discomfort. Expect the vaccine to provide robust protection after the booster – no vaccine guarantees 100% protection, but FeLV vaccines are quite effective in preventing persistent infection.

Possible Side Effects or Risks: Most cats handle the FeLV vaccine well. Common mild side effects include a bit of lethargy or reduced appetite for a day or two, or tenderness at the injection site​. Occasionally, a cat might have a slight fever or feel “off” for a short period – similar to how we might after a vaccine. Serious adverse reactions are uncommon. A small percentage of cats could have an allergic reaction (symptoms like vomiting, facial swelling, difficulty breathing) shortly after the vaccine – this is an emergency, but fortunately rare. One unique concern with feline vaccines, including FeLV, is the very low risk of an injection-site sarcoma (a type of tumor). This is quite rare (estimated roughly 1 case in 10,000 to 30,000 vaccines, though exact rates vary)​. Because of this, vets give the FeLV shot low on a leg, so that in the unlikely event a tumor develops years later, it can be treated effectively. Modern FeLV vaccines have improved formulations (some are recombinant vaccines without adjuvants) to minimize this sarcoma risk​. It’s a risk that exists, but the consensus in veterinary medicine is that the benefit of preventing FeLV – a far more likely and deadly threat – outweighs the tiny chance of a vaccine-related sarcoma​. If you’re worried, talk to your vet about vaccine type; non-adjuvanted vaccines are thought to carry less sarcoma risk. Also, if your cat truly has zero exposure risk as an adult, you can opt not to continue annual boosters – thereby eliminating ongoing vaccine risk – but at least get the kitten series done, since kittens are so vulnerable​.

When to Schedule It: Kittens: All kittens should receive the FeLV vaccine series as part of their initial shots​. Usually the first dose is given around 8 weeks old (sometimes as early as 6 weeks in shelter settings) and a booster ~3-4 weeks later, typically aligning with other kitten vaccines (like their distemper combination). After the booster at about 12 weeks, kittens often get another FeLV shot at around 16 weeks or at their spay/neuter time – protocols vary, but at least two doses are needed. After the kitten series, whether to continue annual FeLV boosters depends on lifestyle. Adult indoor-only cat with no contact with other cats: after their kitten shots, continuing FeLV vaccination may not be necessary​. Some vets will boost at one year of age and then stop if risk is truly zero. Outdoor cats or those that meet new cats: should absolutely get yearly FeLV boosters for life​. Indoor cats in multi-cat homes: If any housemate goes outside or if you occasionally foster cats, it’s safer to keep up the vaccine in all cats, since FeLV could come in through the one that goes out. Also, if your cat is indoors but you might adopt a new cat in the future, it’s wise to maintain protection (new cats should always be tested, but very early infections can be missed). Essentially, schedule an annual exam for your cat and review their FeLV risk each time. If boosters are needed, they’ll be given then. If you decide to discontinue boosters as an adult, that’s fine – just be cautious about future exposures because immunity will wane. And importantly, test cats for FeLV before introducing them to other cats or every year or two if they roam outdoors, even if vaccinated, as no vaccine is 100%.

FAQs or Common Concerns:

• Q: Should I vaccinate my strictly indoor cat for FeLV?
  A: As a kitten – yes, it’s recommended​. As an adult – it depends. Indoor cats have a low risk of contracting FeLV since they aren’t mingling with potentially infected cats. After the initial kitten series (which provides important baseline immunity during their most vulnerable period), many vets make FeLV boosters optional for indoor-only cats beyond one year of age​. If there’s any chance your cat could escape outside, or if you might bring another cat into the home, maintaining FeLV vaccination is a good safety measure​. Consider your environment: do you ever see neighborhood cats nose-to-nose through the screen door? Do you plan to foster or adopt more cats? These scenarios might warrant continued vaccination. If none apply and your cat is solitary indoors, you could reasonably forego annual boosters after kittenhood – but be diligent that they truly have no exposure. Always discuss with your vet; they can tailor the recommendation to your situation.
• Q: How effective is the FeLV vaccine?
  A: It’s quite effective, but not 100%. It greatly reduces the likelihood of persistent infection if a vaccinated cat is exposed. In the past, FeLV was rampant, but thanks in part to widespread kitten vaccination, the prevalence has dropped significantly​. No vaccine guarantees absolute protection – a heavily exposed cat could still possibly get FeLV despite vaccination, but this is uncommon. The vaccine efficacy is high enough that it has been a cornerstone in controlling FeLV in the pet population. Keep in mind: vaccination and avoiding exposure together provide near-total protection. So, a vaccinated indoor cat is extremely safe. An outdoor cat vaccinated is fairly safe, but could still be at some risk especially if constantly exposed – that’s why periodic testing is wise even for vaccinated outdoor cats, and why reducing fights and contact with unknown cats is important.
• Q: If my cat tests positive for FeLV, should I still vaccinate them?
  A: No. If a cat is confirmed FeLV-positive, vaccinating is not useful because they already have the virus (the vaccine cannot eliminate an existing infection)​. At that point, focus on keeping that cat healthy: indoor-only life, good nutrition, and regular vet checkups to manage any symptoms. Also, keep FeLV-positive cats separated from FeLV-negative cats to prevent transmission. The FeLV vaccine is for prevention only, so it’s only given to cats that test negative. In a multi-cat home, you would vaccinate the FeLV-negative cats and not the positive one. And remember to retest that positive cat to confirm (occasionally a test can be falsely positive). Some kittens can fight off FeLV infection after exposure; if one tests positive, a vet might advise isolating and retesting in a few months. Only persistently positive cats are truly infected long-term.

Conclusion: The FeLV vaccine is a lifesaver in the feline world – literally. It has turned what was once a very common deadly infection into a preventable disease. Every kitten should be given this protection early in life​. Whether adult cats continue to need it hinges on their lifestyle and potential exposure to other cats. Pet owners should weigh the very small risks of vaccination against the dire consequences of FeLV infection. In most cases, preventing FeLV is far preferable to dealing with it: FeLV-positive cats often face repeated illnesses, need to be kept isolated from other cats, and have significantly shortened lifespans. By following veterinary guidance on FeLV testing and vaccination, you can spare your cat from that fate. In combination with testing new cats and limiting contact with strays, the FeLV vaccine is a key part of keeping your feline family leukemia-free. Thanks to responsible vaccination, many modern house cats will never know the tragedy of FeLV – and that’s exactly the goal.​