The first experimental infection with FCV 273 induced humoral and cellular crossimmunity against the second challenge virus, the field strain FCV 27, in all cats, but vaccinated cats still had more IFN-γ-releasing PBMCs upon stimulation with FCV 27 before FCV Challenge II with FCV 27 compared to placebo-vaccinated control cats. The gene discussed is IFNG; the disease is infection.