The global horse community has long recognized the necessity of vaccinating against equine influenza (EI). However, immunization protocols are not universal: There is no recognized standard regarding intervals between EI vaccinations.

In an effort to lay scientific groundwork for establishing an international norm, Virology Unit researchers at the Irish Equine Centre in Johnstown, Ireland, evaluated the efficacy of three vaccination schedules: the minimum duration between doses allowed by the Turf Club, Ireland’s Thoroughbred racing authority; the manufacturer’s recommended vaccination schedule; and the maximum permissible dosage intervals according to Irish racing rules.

“Outbreaks of equine influenza can lead to the cancellation of equestrian events and cause severe disruption to the horse industry,” said Ann Cullinane, MVB, PhD, MRCVS, head of the Virology Unit. “Vaccination is crucial to the control of influenza, and decades of implementation of mandatory vaccination in selected populations has proved to be highly effective.

“However, there is often a lack of harmonization between the mandatory vaccination regimes and the vaccine manufacturers’ instructions,” she explained. “Prior to this, there was some concern that vaccination in accordance with the racing regulations but not in accordance with the vaccine manufacturers’ data sheets might leave racehorses susceptible to influenza. It was accepted that the regulations were based on practicality and flexibility rather than science.”

In a randomized clinical trial, the researchers separated 55 unvaccinated, seronegative (showin