Impact of Age at First Immunisation on Equine Influenza Short and Mid‐Term Protective Antibody Levels in Thoroughbred Foals

Reasons for performing study Every year, several equine influenza (EI) epidemics are reported worldwide. Equine influenza vaccination is the most efficient method of prevention; however, not all horses develop sufficient immunity after EI immunisation, increasing the risk of infection, infectious virus excretion and the spread of the disease. Objectives This study aimed to better understand poor vaccine response mechanisms during the primary EI vaccination. Study design Cohort study. Methods The EI humoral immune response was measured in 118 Thoroughbred foals set in 3 different stud farms (SF#1 to SF#3) after the primary course of EI vaccination. All foals were immunised with a recombinant canarypox-based EI vaccine, following the vaccine manufacturer's recommendations. The age at first vaccination was 4 to 8 months. Sera were tested by single radial haemolysis (SRH) against the A/equine/Jouars/4/2006 EIV strain (Florida clade 2) at the time of the first vaccination (V1), 2 weeks and 3 months after the second immunisation (V2), 2 days and 3 months after the third immunisation (V3). Results Short (V2 + 2 weeks) and mid-term (V3 + 3 months) SRH antibody levels were statistically different between the stud farms (P-value = 0.0011 and 0.003, respectively), with SF#1foals at the first immunisation (SF#1 median age 143 days, SF#2 median age 181 days and SF#3 median age 156 days), but were independent of the presence of maternal derived antibodies (MDA; P-value 0.41). Given the results in SF#1, a booster immunisation (V4) was brought forward to restore protective levels of antibody. Conclusion Independently of the presence of MDA, the age of foals at first immunisation plays an important role in the establishment of adequate antibody levels. Ethical animal research: All animal work received ethical approval, and consent was obtained from the owners. Sources of funding: This study was supported by the Basse-Normandie Region council (France) and the European Regional Development Fund. Competing interests: None declared.