Vaccine-Associated Sarcoma: A Possible Side Effect of Vaccinating Your Cat

Updated on January 23, 2020

Vaccine-Associated Sarcoma In Cats

Your four-legged friend, your lap warmer, your fur baby, or your best friend: your cat is more than a pet, they are a part of your family. This is why we start vaccinations as early as six weeks old. From rabies to feline leukemia, we agree to all of the shots to keep our babies happy and healthy for as long as possible.

But what happens when the vaccines cause more harm than good? Cat owners need to educate themselves on the risks that come with vaccinating their cats. Unfortunately, vets do not consider all the facts when pushing unnecessary vaccinations.

There is no disputing that vaccines save lives, but no vaccine is 100% safe either. Vaccinate your pet based on your comfort level. This is our story to increase awareness of vaccine-associated sarcoma and to encourage cat owners not to blindly over-vaccinate their cats.

A happy and healthy Oskar sits with his sister on the bathroom counter.
A happy and healthy Oskar sits with his sister on the bathroom counter.

Oskar's Story

Oskar, our three-year-old American Bobtail, received his yearly check-up three months ago and got his yearly vaccinations for rabies, FVRCP, and feline leukemia. Everything was fine until we returned from a five-day vacation in mid-September to find our baby more lethargic than normal and drastically underweight. Oskar is always very vocal since he is an American Bobtail and to our surprise he whined as we gave him a plethora of cuddles. While petting Oskar, I almost immediately felt a walnut-sized lump on his rear left leg. We panicked and called the vet and set up an appointment to have it examined.

Some $900 later, after two appointments and diagnostics and referral to a specialist, we told he had a Vaccine Associated Sarcoma or VAS. No pet owner wants to receive that news: your baby has an invasive malignant tumor, a tumor caused by the very vaccine you were told to give him to protect him.

It’s now October 25th and the medical bills have crawled over the $3000 mark and that’s just for testing. We got the call this weekend that the CT tests luckily came back showing no metastasis in his lungs or other organs. This means a “simple” amputation should be able to save his life and return his quality of life back to him. We meet with a surgeon tomorrow to begin the process, as the longer we wait, the more the tumor spreads. I don’t want anyone else to have to go through what we are going through, so please continue reading to learn the facts on VAS.

A few weeks later Oskar is too tired to jump higher than the kitchen chair.  He is losing weight and spends all day sleeping.
A few weeks later Oskar is too tired to jump higher than the kitchen chair. He is losing weight and spends all day sleeping.

What Is in Your Vaccine?

One popular vaccine is sold by Merck Animal Health for the prevention of FeLV (feline leukemia virus). This vaccine contains killed viruses that are chemically inactivated with an adjuvant to enhance the immune response. What does this actually mean? It means there is a chemical being injected into your cat that is solely designed to create inflammation at the injection site so the immune system is alerted to its presence. The packaging comes with a warning that “Some reports suggest that in cats, the administration of certain veterinary biologicals may induce the development of injection-site fibrosarcomas.” Most vets do no relay this information to the pet owner so it is our responsibility to research not only the vaccine but the company itself that produced said vaccine.

Are you going to ask you vet more details next time a vaccine is recommended?

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What Is a Sarcoma?

A sarcoma is a highly invasive and aggressive malignant tumor that may appear at the site of injection anywhere for weeks to years after a vaccination. These tumors have microscopic tendrils that reach out from the main tumor site and wrap around blood vessels and muscles making removal very difficult. If not completely removed, there is a 70% chance that cancer will return. The same study showed 97% of cats treated had complete removal but 14% had a re-occurrence and 20% had metastasis to other parts of the body.

With Oskar, the sarcoma appeared on his rear left leg at the injection site and formed a walnut-sized lump. This tumor drastically diminished his quality of life in a short period of time. He went from being a very cuddly cat who ran, leaped, ate and played, to a shadow of himself that dropped to seven pounds and only slept on a low chair since he couldn’t even jump like he used to without being in pain.

Recommended vaccination sites
Recommended vaccination sites | Source


Summit Veterinary Referral



This article is accurate and true to the best of the author’s knowledge. It is not meant to substitute for diagnosis, prognosis, treatment, prescription, or formal and individualized advice from a veterinary medical professional. Animals exhibiting signs and symptoms of distress should be seen by a veterinarian immediately.


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      Lynn Chasan 

      2 years ago

      My cat and I have to move to my sister's house - she has a cat and 3 little dogs how do I do the settling in? I will be in a bedroom with a little balcony - my cat has never used a litter box and has been an only child in her own garden - Im nervous that she will run away - can someone please advise


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