According to the OBP AHS vaccine pamphlet, “Foals born of unvaccinated dams can be inoculated at any age but foals of immune dams should not be vaccinated until they are at least six to seven months old.”
Dr Jacob Modumo, Business Development Officer of Onderstepoort Biological Products (OBP), who manufacture the registered AHS vaccine, assisted with the following clarification: With foals, or horses vaccinated for the first time, there may be serotype “competition” or immune-dominance of some of the serotypes. These more dominant serotypes can suppress the least dominant ones to replicate enough to stimulate an immune response. That is why the vaccination pamphlet advises that “It takes up to 2-3 vaccinations for horses to become immune to all serotypes in the vaccine.”
According to the Animal Diseases Act 35 of 1984, it is compulsory to vaccinate all horses in South Africa except horses in the African horse sickness free zone and the African horse sickness surveillance zone between the ages of 6 and 12 months, then between the ages of 12 and 18 months and then again once every year thereafter with an effective remedy.
After 3 vaccinations, tests have shown that annual vaccination results mostly in sero-conversion that picks up in 6-8 weeks post vaccination and lasts at protective level for a minimum of 10 months. Levels drop below protective level mostly from 11-12 months post vaccination. This is the justification for the recommended annual vaccination.