Strangles vaccination is only recommeneded in "high risk" situations because the vaccine isn't going to prevent disease in most cases and it does carry increased risk over other vaccinations. Boarding situations where horses come and go alot, horses that compete, horses in breeding farm situations, etc are all candidates for vaccination against Strangles. Young horses are also more at risk and so many vets recommend vaccinating weanlings and yearlings.
The intranasal vaccine is a bit more effective at lessening the symptoms of disease and preventing disease than the IM vaccines, but it comes with the increased risk of abcessation at injection sites if given at the same time as other vaccinations.
In 3/4ths of the horses that contract Strangles, a long-lasting immunity develops and can be protective for 5 yrs or more. However, you should also remember that there is a chance that any horse that has had Strangles is harboring the bacteria and shedding it randomly for months to years without showing any symptoms. Horses that have had strangles should also not be vaccinated within a year of having the disease as this greatly increases the risk of severe auto-immune reaction to the disease (purpura hemorhagica).
Licensed Veterinary Technician