Randomized blind trial of a commercial FeLV vaccine.

A randomized blind trial of a commercial FeLV vaccine was conducted to evaluate its performance in cats under conditions of long-term natural exposure. Seventy-nine nonviremic, seronegative cats were randomized into 2 groups. Cats were given 3 doses of either FeLV vaccine or placebo (killed rabies virus vaccine) sc at weeks 0, 3, and 9 of the trial. Six weeks later, 44 known-viremic cats were added to the colony. Cats were housed in a single large room and food dishes and litter pans were used in common. Blood samples were collected at 4, 8, and 12 months after the addition of the viremic cats and were assayed for viremia by use of ELISA. Twelve-month samples were also assayed independently by use of indirect fluorescent antibody testing. Investigators conducted assays on coded samples without knowledge of the cat's vaccination status; neither the investigators nor colony personnel knew which cats had been given the FeLV vaccine and which had been given the placebo until the twelfth month of exposure. After 12 months of cohabitation with infected cats, vaccinated cats had a significantly (P less than or equal to 0.02) lower incidence of persistent viremia (defined as 2 positive ELISA test results at least 8 weeks apart or 1 positive indirect fluorescent antibody test result), compared with the placebo-inoculated cats. The incidence of persistent viremia was approximately 3 times greater among the placebo-inoculated cats than among vaccinates.