The most common method of vaccine administration is by SC or IM injection. This approach is excellent for relatively small numbers of animals and for diseases in which systemic immunity is important. In addition, the veterinarian can be sure an animal has received the appropriate dose of vaccine. However, local immunity is sometimes more important than systemic immunity, and in these cases it is more appropriate to administer the vaccine at the site of microbial invasion. For example, intranasal vaccines protect:
Unfortunately, these techniques require handling each individual animal.
Spraying of vaccines enables the vaccines to be inhaled by all the animals in a herd, group, or flock—an obvious advantage when the unit is large. This method is commonly used in the poultry industry. Alternatively, a vaccine may be administered in feed or drinking water, eg, vaccination of poultry for Newcastle disease and avian encephalomyelitis. Drinking water vaccines are increasingly employed in large swine operations. Fish and shrimp may be vaccinated by immersion in a solution of antigen, which is absorbed through their skin and gills.
Because of the complexity of many disease syndromes or to avoid giving animals multiple injections, it is common to use mixtures of organisms in single vaccines. For example, for bovine respiratory disease complex, combined vaccines are available for bovine respiratory syncytial virus, infectious bovine rhinotracheitis virus, bovine viral diarrhea virus, parainfluenza 3 virus, and Mannheimia haemolytica. Combination vaccines that save considerable time and effort are also commonly used in dogs and cats.
When a mixture of antigens is given to an animal simultaneously, they may compete with one another. However, manufacturers have recognized this and modified vaccines accordingly. Vaccines should never be mixed indiscriminately because one component may dominate and interfere with responses to the other components.
Some veterinarians and owners have expressed concern that the use of vaccine mixtures in this way may somehow “overwhelm” the immune system. This concern is unfounded. The immune system has evolved to respond to complex organisms and multiple simultaneous challenges. The simultaneous administration of multiple vaccines to an animal does not present difficulties to the immune system of normal, healthy animals.
Although it is not possible to devise precise schedules for each vaccine, certain principles are common to all methods of active immunization. Newborn animals are passively protected by maternal antibodies and, in general, cannot be vaccinated until maternal immunity has waned. If stimulation of immunity is deemed necessary at this stage, the mother may be vaccinated during late pregnancy, timing the doses so that peak antibody levels are reached at the time of colostrum formation. It is important to note that modified live vaccines against viruses that cause abortion should not be used in pregnant animals, because the vaccine itself may cause abortion. Neonatal animals are protected against disease caused by that specific pathogen while sufficient maternal antibodies are present. However, passive antibody titers decrease exponentially. These maternal antibodies may drop below protective levels while, at the same time, preventing successful immunization. Inactivated vaccines are not very effective in conferring protective immunity in the face of maternal antibodies. Modified live vaccines, however, may induce a protective primary immune response and some immunologic memory. Because the precise time of loss of maternal immunity cannot be predicted, young animals must usually be vaccinated multiple times to ensure successful immunization, and appropriate biosecurity measures should be used until immunity develops.
The interval between vaccine doses depends on an animal's immunologic memory. The duration of this memory depends on multiple factors, such as the nature of the antigen, the use of live or dead organisms, the adjuvants used, and the route of administration. Some vaccines may induce immunity that persists for an animal's lifetime. Other vaccines may require boosting only once every 2–3 years. Even killed viral vaccines may protect some animals against disease for many years. Unfortunately, the minimal duration of immunity has rarely been reliably measured.
Individual animal and vaccine variability make it difficult to estimate the duration of protective immunity. Within a group of animals, there may be a great difference between the shortest and longest duration of protection. Vaccines may differ significantly in their composition, and although all may induce immunity in the short term, it cannot be assumed that they confer equal long-term immunity. A significant difference likely exists between the minimal level of immunity required to protect most animals and the level of immunity required to ensure protection of all animals.
A veterinarian should always assess the relative risks and benefits to an animal when determining the frequency of revaccination. Owners should be made aware that protection can be maintained reliably only when vaccines are used in accordance with the protocol approved by vaccine licensing authorities. The duration of immunity claimed by a vaccine manufacturer is the minimal duration supported by the data available at the time of approval.
It is now common practice to rate vaccines according to their importance. Essential (or core) vaccines should be given to all animals of a species, and veterinarians should ensure that immunity is maintained throughout an animal's life by appropriate revaccination. Optional (or non-core) vaccines protect animals against sporadic, mild, or uncommon diseases and should only be used when circumstances warrant and when the benefits clearly outweigh the risks involved. For example, essential vaccines in dogs in the USA would normally include canine distemper virus, parvovirus, adenovirus, and rabies virus. Optional vaccines may include canine coronavirus, parainfluenza virus, Bordetella bronchiseptica, leptospirosis, and Lyme disease.
It has long been standard practice to use exactly the same vaccine for boosting an immune response as was used when first priming an animal. However, there is no reason why different forms of a vaccine could not be used for priming and for boosting. This approach is known as a prime-boost strategy. Under some circumstances this may result in significantly improved vaccine effectiveness. Prime-boosting has been most widely investigated in attempts to improve the effectiveness of DNA plasmid vaccines. Combinations usually involve priming with a DNA plasmid vaccine and boosting with either a vectored recombinant vaccine or with adjuvanted protein antigens.
Both natural infection and vaccination induce a protective immune response. As a result, testing an animal’s serum for antibodies may not enable differentiation of infected from vaccinated animals (DIVA). This problem can be resolved by removing a nonprotective antigen from the vaccine. When this is done, testing the animal for antibodies to this specific antigen enables DIVA to occur. Only naturally infected animals will have antibodies to that antigen. A simple serologic assay such as ELISA will provide this differentiation and enable eradication to proceed in the presence of vaccination. An excellent example of this is the eradication of Aujeszky disease (porcine herpesvirus 1) from commercial swine in many developed countries with the aid of DIVA vaccines.
Also see pet health content regarding vaccines and immunotherapy in animals.