However, the combination of FCV F9 vaccination and FCV 273 infection induced a higher cellular immune response compared to FCV 273 infection alone as the median number of IFN-γ-releasing PBMCs was consistently higher in the vaccinated cats compared to the control cats over the whole observation period of FCV Challenge I. Before the second challenge, IFN-γ-releasing PBMCs were observed upon stimulation with FCV 27, and neutralising antibodies were detected in all cats, indicating both humoral and cellular crossimmunity. This evidence concerns the gene IFNG and infection.