Administration of Vaccines

The simplest and most common method is SC or IM injection. This approach is excellent for relatively small numbers of animals and for diseases in which systemic immunity is important. 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 are effective in protecting cattle against infectious bovine rhinotracheitis, cats against feline rhinotracheitis and calicivirus infections, and poultry against infectious bronchitis and Newcastle disease. Unfortunately, these techniques require handling each individual animal. Aerosolization of vaccines enables them 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. Fish and shrimp may be vaccinated by immersion in a solution of antigen, which is absorbed through their gills. Advances in transdermal, needle-free injections make additional routes available.

Mixed Vaccines:

Because of the complexity of many disease syndromes, and 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 (Pasteurella) haemolytica . Mixed vaccines that save considerable time and effort are also commonly used in dogs and cats. Mixed vaccines are often used to protect animals against several different agents with economy of effort, but it may be wasteful to use vaccines against organisms that are not causing problems. In addition, when a mixture of different antigens is inoculated 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. Vaccination Schedules: 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. 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.  Successful active vaccination was previously thought to be possible only after passive immunity had waned. Neonatal animals with certain levels of pathogen-specific, detectable antibodies were thought to be protected against disease caused by that pathogen. However, studies in puppies born to bitches immunized against rabies during pregnancy have shown that passive antibody titers decrease significantly by 6 wk of age. In fact, when challenged with virulent rabies virus at 6 wk of age, >90% of these puppies succumbed. Similar puppies vaccinated at 7 and 11 wk of age with a recombinant vectored rabies virus were all solidly protected against rabies, even though many of them had high levels of maternal antibody when vaccinated. This has also been demonstrated with canine distemper virus and recombinant vaccines. It is now known that an antigen can impart memory to the immune system even when passive maternal antibody is present. With the availability of recombinant vaccines, the age of and interval between vaccinations may have to be reconsidered. Because the exact time of loss of maternal immunity cannot be predicted, young animals are often vaccinated at least twice to ensure successful immunization. The interval between vaccine doses depends on an animal’s immunologic memory. The duration of this memory depends on multiple used, such as the nature of the antigen, the use of live or dead organisms, adjuvants used, and the route of administration. Modern vaccines may induce immunity that persists for an animal’s lifetime. Other vaccines may require boosting only once every 2-3 yr. Even killed viral vaccines may protect some animals against disease for many years. Unfortunately, the minimal duration of immunity has, until recently, rarely been reliably measured. Annual revaccination has been the rule because this approach is administratively simple and has the advantage of ensuring that an animal is regularly seen by a veterinarian. It is likely that this is more than sufficient for most vaccines. Individual animal and vaccine variability make it difficult to estimate the duration of 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 longterm 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. Unfortunately, there is insufficient information available for most vaccines to determine minimal vaccination intervals. 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 that is supported by the data available at the time of approval.