Experience of the efficacy of equine influenza vaccinations.

In 1974, a very extensive influenza/A/equi 2 epidemic broke out in the Finnish horse population. To study the efficacy of influenza vaccinations a questionnaire was sent after the epidemic to all Finnish veterinarians. The answer material was selected to contain only stables which had had clinically typical cases. The material consisted 234 unvaccinated and 629 vaccinated horses. In the latter group 466 horses were vaccinated adequately. The results of the study showed that of the unvaccinated horses 212 (91%) and of the adequately vaccinated horses only 42 (9.4%) contracted clinically typical influenza. On the other hand, in the group of horses vaccinated irregularly or only once before the epidemic the disease prevalence was about the same as for the unvaccinated animals. Thus, the equine influenza vaccines used in Finland had a surprising good efficacy if they were used according to instructions.