Feline Leukemia Vaccine

Keeping cats confined indoors in a feline leukemia virus (FeLV) free household is the simplest form of protection against FeLV infection.  For cats allowed to wander outside, the feline leukemia vaccine (FeLV) provides the best means of protection, however they are not 100% effective. Effectiveness of vaccines can range between 20-100%, depending on the particular study.  Any cat receiving a FeLV vaccine should be tested for FeLV infection prior to vaccination. Click here to learn more about testing for feline leukemia.

Types of feline leukemia vaccines

Killed whole cell vaccines tend to be the most effective at providing protection from FeLV infection.  These vaccines use whole killed virus preparations or subunits of the virus (gp70 subunit).  Modified live vaccines have been avoided due to concerns that the live virus might insert itself into the cat’s DNA and cause disease later in life.  This is due to the ability of FeLV to insert itself in to a cat’s DNA, in apparently recovered cats exposed to the virus, and cause FeLV related disease years later.  These cats are called latent carriers of FeLV virus because they test negative for FeLV with traditional tests for the virus, but test positive with more sensitive tests such as polymerase chain reaction (PCR) testing. 

A canarypox-FeLV recombinant vaccine without adjuvant has been developed that has similar effectiveness in protection against FeLV infection.  This vaccine was developed due to concerns of some cats developing a tumor at the site of FeLV vaccination.  This tumor, called vaccine-associated sarcoma, is associated with administration of FeLV and rabies vaccines and is estimated to occur in 1-3/10,000 vaccines administered.  The canarypox-FeLV recombinant vaccine is thought to less likely cause this type of tumor in a cat since it does not contain adjuvant.  An adjuvant is used in vaccines to help stimulate an immune response to the vaccine.  Although it is uncertain what causes these tumors to form, the adjuvants used in vaccines are thought to play some role in their development. Due to concerns of this type of tumor, FeLV vaccines should be administered in the lower left rear leg region of the cat.

Recommendations regarding feline leukemia vaccination in cats

General recommendations for vaccinating cats for FeLV are that all kittens be vaccinated with two doses of vaccines beginning as early as 8-9 weeks and 12 weeks of age then booster at 1 year of age after the completion of the initial series.  There is no consensus on vaccination of cats for FeLV after 1 year of age.  Since duration of protection beyond 1 year is unknown, it has been recommended that adult cats at risk of exposure (i.e. outdoor cats) be vaccinated annually.  Due to concerns of vaccine-induced tumors and uncertainty of duration of protection from the FeLV vaccines, some veterinarians advise booster FeLV vaccination every 2-3 years for at risk cats.  However, booster vaccination is not recommended for all cats, particularly indoor cats.

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Dr Stephen Atwater

Stephen W. Atwater, DVM, MS, Diplomate ACVIM (oncology) is a board-certified veterinary oncology specialist. His professional interests include utilizing emerging therapies for difficult to treat cancers.

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