Equine influenza (EI) is a highly contagious respiratory disease. This important pathogen for the equine industry has been responsible for numerous outbreaks worldwide and is still actively circulating. Vaccination is the primary tool for prevention against this virus. However, in some cases the vaccine, although necessary, may have limitations, for example during the 2019 outbreak in Europe. The occurrence of this outbreak led to a change in the vaccination protocol by the WOAH (World Organisation for Animal Health) to semi-annual vaccination. The aim of this study was to compare Single Radial Haemolysis (SRH) antibody levels in response to a vaccine boost with a whole inactivated iscomatrix-adjuvanted equine influenza vaccine, administered alone or combined with the inactivated parapoxvirus ovis (iPPVO) treatment (3 injections of iPPVO at D0, D2 and D4 post vaccination). Each group contains 10 adult horses with regular annual EI vaccination. This study required a vaccine boost 6 months after the last immunisation. Antibody levels were measured with the SRH assay at 1, 3 and 6 months post-vaccination. Results revealed that horses that received a iPPVO/EI vaccine co-injection had significantly higher antibody levels than the group that received the EI vaccine alone both in magnitude and duration. Although the vaccine used contained only a clade 1 and a European lineage strain, the increase in protective SRH antibodies was observed for both a clade 1 and a clade 2 strain. In conclusion, this study showed that the use of iPPVO as an immunostimulant could increase the level of protective antibodies measured in SRH and extend the duration of protection, when co-injected with an equine influenza vaccine.