The difficulty of developing efficacious vaccines against lentiviruses, such as HIV-1 and FIV, has long been recognized and may be related to the number of subtypes and diversity of strains within each subtype. Concern has been raised that the commercially available FIV vaccine (Fel-O-Vax® FIV-Fort Dodge Animal Health, Fort Dodge, IA) might not be efficacious against the most common strain in the United States, subtype B. This article reports on 2 studies to determine if there was cross-protection with the commercial vaccine. The first study tested the vaccine against a virulent subtype B strain from an FIV-infected cat. Ten specific pathogen-free cats received Fel-O-Vax FIV while control cats were given either uninfected vaccine cells or phosphate-buffered saline. Three weeks after immunization, the cats were intravenously challenged with virus from the infected cat. The vaccinated cats have remained FIV negative for 1 year, with normal CD4 counts and normal CD4-CD8 ratios. The control group became infected within 6 weeks after inoculation. In the second study, 4 specific pathogen-free cats were immunized with Fel-O-Vax FIV and 3 cats were given phosphate-buffered saline. The cats were challenged using the same inoculum, dose, and schedule as in the first study. All vaccinated cats were protected and had normal CD4 counts and normal CD4-CD8 ratios at 18 weeks after inoculation (the length of the study), while the control cats were infected by 9 weeks and had substantially decreased CD4 counts and CD4-CD8 ratios. The Fel-O-Vax FIV vaccine did provide 100% protection against the subtype B, which is the predominant subtype in the United States.

COMMENTARY: This FIV vaccine contains the subtypes A and D. The vaccine was approved based on challenge studies with subtype A. This additional challenge work with subtype B should ease the minds of practitioners concerned about its efficacy in the United States.

Dual-subtype FIV vaccine (Fel-O-Vax® FIV) protection against a heterologous subtype B FIV isolate. Pu R, Coleman J, Cosiman J, et al. J FELINE MED SURG 7:65-70, 2005.