Both horses and humans are very susceptible to tetanus, horses even more so than humans and much more so than other species of animals. In humans, there are many studies examining the duration of immunity following vaccination, so physicians can feel confident in their recommendations of extending the revaccination interval. In horses, a 6 month study comparing responses to commercial vaccines demonstrated significant antibody response for the duration of the study, but did not measure the response beyond 6 months. (There have not been any studies in North America that have challenged vaccinated horses with tetanus toxoid, but antibodies alone can mediate protection.) Conclusions about efficacy of the vaccine and duration of protection are based on antibody levels detected in laboratory studies combined with field experience, and the recommendation for yearly vaccination stems from this. There have been rare reports of horses with clinical cases of tetanus that had been vaccinated, however survival of horses with tetanus is strongly associated with previous vaccination. More specifically, survival is especially associated with vaccination against tetanus within the year prior to sustaining a cut or laceration that puts them at risk for the disease.
Vaccination FAQ - Why are horses vaccinated for tetanus yearly, whereas humans are boostered every 5-10 years?