Success and limitation of equine influenza vaccination: the first incursion in a decade of a Florida Clade 1 equine influenza virus that shakes protection despite high vaccine coverage

International audienceEvery year, several epizooties of equine influenza (EI) are reported worldwide. However, no EI case has been identified in France between 2015 and late 2018, despite an effective field surveillance of the pathogen and the disease. Vaccination against equine influenza virus (EIV) remains to this day one of the most effective methods to prevent or limit EI outbreaks and the lack of detection of the pathogen could be linked to vaccination coverage. Equine influenza immunity and vaccine coverage in France was evaluated through a large-scale serological study, with more than 300 serums samples analysed. Results indicate that overall immunity coverage in early 2018 against EIV infection reach 87.6%. DIVA analysis indicates that EI-specific immune response was likely induced by EI vaccination alone, which support the absence of EIV circulation in French horse populations between 2015 and late 2018, as reported by the French equine infectious diseases surveillance network (RESPE). Despite this strong EI immunity, the incursion in the EU of a Florida Clade 1 sub-lineage EIV (undetected in France since 2009) induces several EI outbreaks in France from December 2018 to July 2019. However, the benefit of a high vaccine coverage was noticeable. Clinical signs of disease were mild when observed in vaccinated horses (with anecdotal reports of death in naïve horses) and the disease was effectively kept under control when the virus was introduced in vaccinated herds