Concepts of Herd level Immunity

The herd-level duration of immunity can be impacted by many things, and in general, immunity resulting from natural exposure and infection is different from immunity resulting from vaccination. Immunity resulting from natural exposure and infection is often based on experimental studies or field experience with particular strains or subtypes of a virus, for example. Vaccine-induced immunity is often based on the revaccination schedule recommended by the vaccine manufacturer, which depends on the duration of effective protection for that vaccine. This in turn depends on the specific antigen content, type of vaccine (live or dead, for example), and the route of vaccine administration. Protection against an infectious disease can only be maintained reliably when vaccines are used in accordance with the protocol approved by the vaccine-licensing authorities. The duration of immunity claimed by the vaccine manufacturer is the minimum duration of immunity that is supported by the data that is available when the vaccine is licensed. It is important to note that not every animal in a herd will respond the same to vaccination, even if the vaccine is always administered correctly. Animals may fail to respond to vaccination due to prior passive immunization, immunosuppression, biological variation, or inadequate vaccine. An animal may respond to a vaccine but the vaccine may be given too late (i.e. the animal is already infected), include the wrong strain or organism, or the antigens included may not be protective (preventing clinical signs but not viral shedding, for example). As a result, in any population of animals, we can expect that protective immune responses following vaccination will be normally distributed such that some animals produce a poor immune response resulting in poor protection. (Source: Tizard I., 2009. Veterinary Immunology: An Introduction. Saunders, St Louis, MO.)