In the UK the estimated population of horses is in the region of 1.1 million, most of which are leisure horses with the majority in single horse ownership. Most owners do not think of their individual horse as part of a larger population of horses either at yard level or even in a local region. Your horse is part of a herd whether this is their field companion, the other horses on the yard or even the other horses in your local area. This means that your horse may be at risk of picking up an infectious disease even if it doesn’t leave home and could contribute to the spread of disease to other susceptible horses.
Herd immunity is a form of immunity that occurs when vaccination of a significant proportion of the herd (usually > 85%) provides a measure of protection for individuals who have not been vaccinated or have not developed immunity through natural exposure. When a sufficient proportion of the herd (> 85%) has been vaccinated the disease is unable to spread because there are too few susceptible horses left to propagate the outbreak. This means that outbreaks of a disease can be controlled without the need for all horses to be vaccinated.