Immunogenicity of Calvenza-03 EIV/EHV® Vaccine in Horses: Comparative In Vivo Study

Equine influenza (EI) is a highly contagious acute respiratory disease of equines that is caused mainly by the H3N8 subtype of influenza A virus. Vaccinating horses against EI is the most effective strategy to prevent the infection. The current study aimed to compare the kinetics of EI-specific humoral- and cell-mediated immunity (CMI) in horses receiving either identical or mixed vaccinations. Two groups of horses were previously (six months prior) vaccinated with either Calvenza 03 EIV EHV(R) (G1) or Fluvac Innovator(R) (G2) vaccine. Subsequently, both groups received a booster single dose of Calvenza 03 EIV EHV(R). Immune responses were assessed after 10 weeks using single radial hemolysis (SRH), virus neutralization (VN), and EliSpot assays. Our results revealed that Calvenza-03 EIV/EHV(R)-immunized horses had significantly higher protective EI-specific SRH antibodies and VN antibodies. Booster immunization with Calvenza-03 EIV/EHV(R) vaccine significantly stimulated cell-mediated immune response as evidenced by significant increase in interferon-gamma-secreting peripheral blood mononuclear cells. In conclusion, Calvenza-03 EIV/EHV(R) vaccine can be safely and effectively used for booster immunization to elicit optimal long persisting humoral and CMI responses even if the horses were previously immunized with a heterogeneous vaccine.