Vaccines play an essential role in keeping cats healthy, protecting them from serious and potentially life-threatening diseases such as rabies and feline leukemia virus. For the vast majority of cats, vaccines are safe, effective, and lifesaving. In rare cases, however, a small number of cats can develop a cancer known as Vaccine-Associated Sarcoma (VAS) at the site of an injection.

Although uncommon, VAS is an aggressive tumor, and early awareness truly matters. Knowing what changes are expected after vaccination, recognizing when a lump needs veterinary attention, and understanding how these tumors are treated can help cat owners feel informed and empowered to act early if concerns arise. Here are the answers to some of the most common questions about Vaccine-Associated Sarcoma in cats from Dr. Sarah Charney, one of our board-certified oncologists, and co-owner of Boundary Bay Veterinary Specialty Hospital.

Injection-Site Cancer in Cats: What Is Vaccine-Associated Sarcoma?

Vaccine-Associated Sarcoma (VAS) is most commonly a fibrosarcoma, a malignant tumor that arises from connective tissues such as fibrous tissue or muscle. These tumors develop at the site of a previous injection—most often a vaccine—although other injectable medications have also been associated with similar sarcomas.

VAS tumors tend to be locally aggressive, meaning they grow into surrounding tissues rather than spreading quickly to distant organs. This growth pattern is why early detection and careful treatment planning are so important.

Why Can Vaccines Trigger Sarcoma in Some Cats?

Some vaccines contain adjuvants, such as aluminum, which are used to stimulate a stronger immune response. In most cats, this results in temporary, mild inflammation at the injection site.

In a very small number of cats, inflammation may persist for a prolonged period. Over time, this chronic inflammation is believed to increase the risk of abnormal cell growth, which can eventually lead to sarcoma formation. It is important to emphasize that this is a rare complication, and vaccines remain a cornerstone of feline preventive health.

How Common Is Vaccine-Associated Sarcoma in Cats?

Vaccine-Associated Sarcoma is estimated to occur in approximately 1 to 10 cats per 10,000 vaccinated. While the overall risk is low, these tumors are often locally invasive, which means they can be challenging to treat if not addressed early. Prompt evaluation of suspicious lumps can significantly improve treatment options.

Lumps After Vaccination: When Should You Be Concerned?

Not every lump that develops after a vaccination is cancer. Mild swelling or firmness at an injection site can be a normal reaction and often resolves within a few weeks.

Veterinarians use the 3-2-1 rule to help determine when a lump should be checked:

• The lump is still present three weeks after vaccination
• The lump is larger than two centimeters (about the size of a marble)
• The lump continues to grow one month after the injection

If a lump meets any of these criteria, your cat should be examined by a veterinarian as soon as possible.

How Is Vaccine-Associated Sarcoma Diagnosed?

If VAS is suspected, your veterinarian may recommend consultation with a veterinary oncologist to guide next steps.

Sampling the Tumor

Diagnosis usually begins with a fine needle aspiration (FNA) or biopsy. When sarcoma is suspected, an incisional biopsy—sampling only part of the tumor—is often preferred. This preserves future surgical options, as complete tumor removal requires wide margins of healthy tissue.

Imaging

Imaging helps define how far the tumor extends and supports treatment planning:

• X-rays and ultrasound may be used to assess overall health and look for evidence of spread.
• CT scans are commonly used to map the tumor’s size and depth prior to surgery.

Bloodwork

Routine blood tests help assess overall health and ensure your cat is a good candidate for anesthesia and treatment.

Treatment Options for Vaccine-Associated Sarcoma in Cats

Because Vaccine-Associated Sarcoma is highly invasive, treatment must be carefully planned and aggressive to reduce the risk of recurrence. The best outcomes are typically achieved using a multimodal approach, combining more than one form of therapy.

Surgery

Surgery is the most important component of treatment.

• The goal is to remove the tumor with wide margins of healthy tissue, usually at least 3–5 cm in all directions.
• Incomplete removal is associated with a very high recurrence rate, reported to be as high as 90%.
• Advanced imaging is often used to guide surgical planning and improve the likelihood of complete removal.

Radiation Therapy

Radiation therapy is frequently used alongside surgery to improve local tumor control.

• Before surgery, radiation may help shrink the tumor and make removal easier.
• After surgery, radiation targets microscopic cancer cells that may remain, lowering the risk of regrowth.

Chemotherapy

Chemotherapy may be recommended in selected cases.

• It can help slow tumor progression or address microscopic disease.
• Commonly used drugs include doxorubicin, cyclophosphamide, and carboplatin.
• Not all cats need chemotherapy, and recommendations are made on an individual basis.

What Is the Prognosis for Cats With Vaccine-Associated Sarcoma?

Prognosis depends on tumor size, location, and how early treatment begins. Veterinary oncology data show improved survival times when aggressive, combined treatment is pursued rather than surgery alone.

Reported averages include:

• Surgery alone: about 7 months
• Surgery plus radiation: about 1.5 years
• Surgery, radiation, and chemotherapy: 2 years or longer

Early detection and comprehensive care offer the best opportunity for long-term tumor control and a good quality of life.

Reducing Risk While Continuing to Vaccinate

Vaccines remain essential for protecting cats from serious and potentially life-threatening diseases. When it comes to Vaccine-Associated Sarcoma, the goal is risk reduction—not vaccine avoidance.

Several strategies are used to help lower risk while still providing appropriate protection:

• Use non-adjuvanted vaccines when appropriate. Some vaccines are formulated without inflammatory adjuvants. Your veterinarian can help determine which options are safest for your cat.
• Administer vaccines in recommended locations. Vaccines are no longer given between the shoulder blades. Instead, specific sites are used so that, if a tumor were ever to develop, surgical treatment would be more feasible:
  • Rabies vaccine: right rear leg
  • Feline leukemia (FeLV) vaccine: left rear leg
  • FVRCP vaccine: lower right shoulder
• Avoid unnecessary vaccines. Not every cat needs every vaccine every year. Vaccination schedules should be tailored to a cat’s age, lifestyle, and risk factors.
• Monitor injection sites regularly. Check vaccination sites at home and report any persistent or growing lumps to your veterinarian promptly.

Working with your veterinarian to create an individualized vaccination plan is the best way to balance disease prevention with long-term safety.

Oncology Care at BBVSH Bellingham

BBVSH Bellingham provides advanced veterinary oncology care with a focus on individualized treatment planning, medical oncology services, and close collaboration with referring veterinarians and specialty partners.

Our board-certified oncologists and team supports families through diagnostics, chemotherapy, and coordination of surgery or radiation therapy when needed, always prioritizing comfort, quality of life, and clear communication.

Getting Started

Cats are most often seen through referral from a primary care veterinarian, which allows medical records to be shared efficiently. If your cat does not currently have a regular veterinarian, pet owners may contact BBVSH Bellingham directly to discuss next steps.

Key Takeaways for Cat Owners

Vaccine-Associated Sarcoma is rare, but it is a serious condition that benefits greatly from early recognition and prompt treatment.

• Follow the 3-2-1 rule for post-vaccine lumps
• Monitor injection sites regularly
• Seek veterinary evaluation early
• Specialized oncology care can improve outcomes

Your cat’s comfort, health, and quality of life matter. If you have concerns about vaccines, injection-site lumps, or cancer risk, your veterinary team is here to help.

Adapted from a presentation by Sarah Charney, DVM, DACVIM (Oncology), DACVR

 

Frequently Asked Questions About Vaccine-Associated Sarcoma in Cats

What is vaccine-associated sarcoma (VAS) in cats?

Vaccine-Associated Sarcoma is a rare cancer that can develop at the site of an injection in cats. It is most commonly a fibrosarcoma, a malignant tumor that arises from connective tissues such as muscle or fibrous tissue. These tumors are locally aggressive and require prompt veterinary evaluation.

How common is vaccine-associated sarcoma in cats?

VAS is uncommon, affecting an estimated 1 to 10 cats per 10,000 vaccinated. While the risk is low, these tumors can grow into surrounding tissues, which is why early detection and treatment are important.

What causes vaccine-associated sarcoma in cats?

Some vaccines contain adjuvants, such as aluminum, which help stimulate the immune system. In most cats, this causes only short-term inflammation. In a very small number of cats, inflammation may persist over time, increasing the risk of abnormal cell growth that can lead to sarcoma formation.

Should I stop vaccinating my cat because of VAS?

No. Vaccines are a critical part of preventive care and protect cats from serious and potentially life-threatening diseases. The goal is risk reduction, not vaccine avoidance, by using appropriate vaccine types, recommended injection locations, and individualized vaccination schedules.

What is the 3-2-1 rule for vaccine lumps in cats?

The 3-2-1 rule helps determine when a lump should be checked by a veterinarian:

• The lump is still present 3 weeks after vaccination
• The lump is larger than 2 cm
• The lump continues to grow 1 month after the injection

Any mass that meets one or more of these criteria should be evaluated promptly.

How is vaccine-associated sarcoma diagnosed?

Diagnosis usually begins with a fine needle aspiration (FNA) or biopsy. When sarcoma is suspected, an incisional biopsy is often preferred. Imaging such as X-rays, ultrasound, or CT scans may be used to assess tumor extent and plan treatment, along with routine bloodwork.

What treatment options are available for cats with vaccine-associated sarcoma?

Treatment typically involves a multimodal approach, which may include:

• Aggressive surgery with wide margins
• Radiation therapy before or after surgery
• Chemotherapy in select cases

Treatment plans are tailored to each cat based on tumor behavior and overall health.

Why does surgery for vaccine-associated sarcoma require wide margins?

VAS tumors are highly invasive at the local level. Incomplete removal is associated with recurrence rates as high as 90%, which is why surgery must include wide margins of healthy tissue to improve long-term control.

What is the prognosis for cats with vaccine-associated sarcoma?

Prognosis depends on tumor size, location, and how early treatment begins. Published veterinary oncology data show improved outcomes with combined treatment:

• Surgery alone: about 7 months
• Surgery plus radiation: about 1.5 years
• Surgery, radiation, and chemotherapy: 2 years or longer

Early detection offers the best chance for long-term tumor control and quality of life.

Can vaccine-associated sarcoma be prevented?

VAS cannot be completely prevented, but risk can be reduced by:

• Using non-adjuvanted vaccines when appropriate
• Vaccinating in recommended locations
• Avoiding unnecessary vaccines
• Monitoring injection sites regularly

An individualized vaccination plan is the best way to balance protection and safety.

When should my cat see a veterinary oncologist in Bellingham?

If a lump is suspicious, diagnosed as cancer, or suspected to be vaccine-associated sarcoma, referral to a veterinary oncologist is recommended. Oncology specialists can help guide diagnostic testing and develop an appropriate treatment plan.

How can I access oncology care at BBVSH Bellingham for my cat?

Cats are most often seen by the oncology service at Boundary Bay Veterinary Specialty Hospital – Bellingham through referral from a primary care veterinarian, which allows medical records to be shared efficiently. If your cat does not currently have a regular veterinarian, pet owners may contact BBVSH Bellingham directly to discuss next steps and available options.