Feline Leukemia Virus Vaccine

From AVMA Feline Vaccines:  Benefits and Risks

Feline leukemia virus (FeLV) is the leading viral killer of cats. The virus is spread from cat-to-cat through bite wounds, through casual contact with infected cats, and from an infected mother cat to her kittens. The individuals most at risk of infection are outdoor cats, indoor/outdoor cats, and cats exposed to such individuals. Cats living in households with FeLV-infected cats or with cats of unknown infection status are also at risk. Indoor-only cats with no exposure to potentially infected cats are extremely unlikely to become infected. FeLV vaccines are recommended for all cats at risk of exposure to the virus.