Active immunization involves administration of vaccines containing antigenic molecules (or genes for these molecules) derived from infectious agents. In response, the animals mount adaptive immune responses and develop prolonged, strong immunity to those agents. Vaccines are by far the most effective way of controlling infectious diseases and, as a result, have increased human and companion animal longevity and made intensive livestock production possible.
Several criteria determine whether a vaccine can or should be used. First, the actual cause of the disease must be determined. Although this seems self-evident, it has not always been followed in practice. For example, although Mannheimia haemolytica can be isolated consistently from the lungs of cattle with respiratory disease, these bacteria are not the sole cause of this syndrome, and vaccines against the primary viral pathogens are required for full protection. In some important viral diseases, such as equine infectious anemia, feline infectious peritonitis, and Aleutian disease in mink, antibodies may contribute to the disease process, and vaccination may therefore increase disease severity.
An ideal vaccine for active immunization should confer prolonged, strong immunity in vaccinated animals and induce rapid onset of immunity. Ideally, depending upon the pathogen, it should induce the most effective response, such as type 1 or type 2 immunity depending upon the nature of the pathogen. It should preferably stimulate responses distinguishable from those due to natural infection so that vaccination and eradication may proceed simultaneously. Vaccination is not always an innocuous procedure; adverse effects can and do occur. Therefore, all vaccinations must be governed by the principle of informed consent. The risks of vaccination must not exceed those caused by the disease itself.
Also see pet health content regarding vaccines and immunotherapy in animals.