Influential factors inducing suboptimal humoral response to vector-based influenza immunisation in Thoroughbred foals.

Context: Numerous equine influenza (EI) epizooties are reported worldwide. EI vaccination is the most efficient methods of prevention. However, not all horses develop protective immunity after immunisation, increasing the risk of infection and transmission.

Objectives: This field study aimed to understand the poor response to primary EI vaccination.

Study Design: The EI antibody response was measured in 174 Thoroughbred foals set in 3 stud farms (SF#1 to SF#3) over a 2years period. All foals were immunised with a commercial recombinant canarypox-based EI vaccine. Sera were tested by single radial haemolysis against the A/equine/Jouars/4/06 EIV strain (H3N8) at the time of the first vaccination (V1), 2weeks and 3months after the second immunisation (V2), 2days and 3months after the third immunisation (V3).

Results: The frequency of poor-responders (no detectable antibody titres) was surprisingly elevated after V2 (56.8%), increased to 81.7% at V2+3months and reached 98.6% at V3. The frequency of poor-responder was still 19.2%, 3months after V3. Two independent influential factors were identified. The short (V2+2weeks) and mid-term (V2+3months, V3+3months) antibody levels were positively correlated to the age at V1 (p-value=0.003, 0.031 and 0.0038, respectively). Presence of maternally-derived antibodies (MDA) at V1 was negatively correlated with antibody levels after V3 only (p-value=0.0056). Given that SF#1 antibody response was below clinical protective levels at all-time points studied, the annual boost immunisation (V4) was brought forward by 7.0±1.1months. V1 was delayed by 7weeks the following year, which significantly increased short- and mid-term antibody titres (p-value=9.9e-07 and 2.31e-07, respectively).

Conclusion: The age and MDA at first immunisation with the canarypox-based IE vaccine play an independent role in the establishment of antibody levels. This study also highlights the benefit provided by serological surveillance to evaluate herd immunity and to implement corrective management/vaccination measures.