Equine influenza vaccine containing older H3N8 strains offers protection against A/eq/South Africa/4/03 (H3N8) strain in a short‐term vaccine efficacy study

Summary Reasons for performing study : Surveillance of equine influenza viruses has suggested that strains included in currently licensed vaccines are a poor match for those predominantly circulating in the field. Objective : To assess the ability of Duvaxyn IE‐T Plus to provide cross protection against the newly evolved South Africa/4/03 (H3N8) strain of equine influenza virus. Methods : The vaccine efficacy was evaluated by challenge infection with influenza strain A/eq/South Africa/4/03 (H3N8) 2 weeks after a primary course of 2 vaccinations with Duvaxyn IE‐T Plus given at a 4‐week interval. The outcome of challenge in vaccinated ponies was compared with that in unvaccinated animals. Results : At the time of challenge, all vaccinated ponies had high levels of antibody to Newmarket/1/93, Newmarket/2/93 and South Africa/4/03 strains measured by single radial haemolysis. After challenge infection, there were statistically significantly decreased clinical scores and virus shedding was significantly lower in the vaccinated ponies compared to unvaccinated controls. Conclusion : Two doses of Duvaxyn IE‐T Plus provides good clinical and virological protection against challenge with a variant virus 2 weeks after the 2 doses of vaccine. Potential relevance : When variant strains of equine influenza virus first emerge, booster immunisations with currently available vaccines may limit infection provided sufficiently high antibody levels are achieved, suggesting that vaccination in the face of an outbreak may be beneficial.