FIV is commonly classified into five different subtypes (A, B, C, D, and E). Subtypes A and B are the predominant subtypes in the United States.

Fel-O-Vax® FIV is an inactivated, dual subtype (based on strains of Petaluma subtype A and Shizuoka subtype D) feline immunodeficiency virus (FIV) vaccine.

Currently available antibody-based FIV diagnostic tests (e.g., SNAP® Feline Combo, PetChek® FIV Ab plates, and Western blot) available in the United States and Europe cannot distinguish cats vaccinated with Fel-O-Vax® FIV from FIV-infected cats or from cats that are both vaccinated and infected. Most shelters and other facilities designed to house strays often euthanize cats with positive FIV test results, so previously vaccinated uninfected cats may needlessly undergo euthanasia.