Vaccine-associated Sarcomas in Cats

What are vaccine-associated sarcomas?

Vaccine-associated sarcomas (VAS) were first recognized in 1985 when adjuvanted vaccines for cats were first introduced.  Killed vaccines have no live virus to stimulate the immune system.  Adjuvants are added instead. Traditional feline leukemia and rabies vaccines are both killed vaccines.

Long-lasting injections (antibiotic, steroid) and tissue trauma with injections have also been identified as predisposing causes.


Why are VAS found in cats versus dogs?

Cats are more sensitive to oxidative injury and chronic inflammation than many species, including dogs.  Free radical damage from adjuvants can cause mutations that may lead to sarcoma development.  These mutations may occur several inches away from the initial site of vaccination.  Chronic inflammation has also been seen to cause ocular (eye) tumors in cats after foreign body penetration and trauma.


No virus or significant genetic predisposition (only in a few cases) has been identified.


How common is VAS?

Studies have ranged from VAS developing in 1 in 1000 to 1 in 10,000 cats. It is much rarer in dogs and ferrets.


When and where would a VAS develop?

The time from vaccination to tumor development may be as early as 3 months up to 4 years or more.


In 1996, the site of vaccination was changed from the scruff area to the legs.  The right front leg – distemper vaccine, the right rear leg – rabies vaccine, and the left rear leg – leukemia vaccine.


My cat has a lump after vaccination. What do I do?

Local vaccine reactions can occur 1-2 weeks after the injection.  To help differentiate between a post-vaccine lump and a sarcoma, the VAS Task Force has developed the 3, 2, 1 rule:

3 – if the lump is present 3 months post-vaccination or

2 – if the lump is larger than 2 centimeters (approximately 1 inch) or

1 – If the lump increases size after 1 month of vaccination then biopsy with histopathology should be performed to diagnose the lump.


What is the treatment for VAS?

If a post-vaccination lump is diagnosed as a sarcoma, staging to look for metastasis followed by radical surgery, radiation, and chemotherapy are done to increase disease-free survival time.


Given the high cost of treatment and relatively poor prognosis with VAS, prevention is desirable.  Vaccines are still vital in preventing life-threatening diseases in the majority of cats.  Non-adjuvanted vaccines have been developed.  We carry only non-adjuvanted feline leukemia and rabies vaccine (Purevax) to cats. The Purevax rabies vaccine comes in a 1 year and 3 year duration.  The 1 year vaccine is typically well tolerated.  Patients receiving the 3 year vaccine have been reported with increased soreness at the vaccine site, lethargy, and decreased appetite which typically resolves without treatment within 24 hours.