Basic reproduction number for equine-2 influenza virus a (H3N8) epidemic in racehorse facilities in Japan, 1971

Characterizing the epidemiologic determinants of equine influenza to facilitate effective control measures, including determining the critical proportion of the population that must be immunized to achieve containment, is critical. Through the use of counting process and martingale method, the basic reproduction number (R₀; a measure of transmission potential) of equine-2 influenza A virus (H3N8) infection was estimated based on outbreak data recorded in 1971. The final attack rate ranged from 81.9% to 99.4%. Our estimate of R₀ was in the order of 2 to 5, which was smaller than the previously suggested estimate. Numerical analysis supported airborne spread as the major mode of transmission. Given that simple threshold vaccination coverage might not be realistic within racing stables, the combination of movement restrictions, isolation, and other countermeasures alongside vaccination is recommended to contain outbreaks of equine influenza.