Vaccines are imperfect. They are never 100% effective. Even when administered appropriately, they will not induce immunity in every animal that receives the vaccine. This simply reflects biological variation within the vaccinated animal population. In analyzing the reasons why a vaccine may fail to protect, it is possible to attribute this to one of three different problems. One may blame the vaccine, the vaccinator, or the animal. It may be that the vaccine is to blame. For some reason it failed to work—vaccine failure. Alternatively, perhaps the vaccine was just fine, and the lack of response was the animal’s fault as the animal simply failed to respond—animal failure. Or perhaps the vaccine was not administered correctly—administration failure (Fig. 9.1). At least some cases of vaccine failure result from unsatisfactory or incomplete administration of the vaccine or noncompliance with manufacturer’s recommendations. For example, a live vaccine may have died as a result of poor storage, the use of antibiotics or disinfectants in conjunction with live bacterial vaccines, the use of chemicals to sterilize the syringe, or the excessive use of alcohol swabs on the skin. Sometimes animals given vaccines by unconventional routes may not be protected. When large flocks of poultry or mink are to be vaccinated, the vaccine can be administered as a spray or in drinking water. If the spray is not evenly distributed throughout a building, or if some animals do not drink, some may fail to receive sufficient vaccine. Premature vaccination of young animals before loss of maternal antibodies remains a problem. Animals that subsequently develop disease may be interpreted as cases of vaccine failure. Even animals given an adequate dose of an effective vaccine may fail to be protected if the vaccine is given at the wrong time. Animals require several days after vaccination before immunity develops. If an animal is already incubating the infection, clinical disease may develop and this may be interpreted as either vaccine failure or attributed to the vaccine virus. For automated procedures such as those employed in the poultry and fish industries it is essential to ensure that the automatic injectors are functioning correctly. For obese animals, injection into adipose tissue may drastically reduce vaccine efficacy. Vaccines may be administered by the wrong route, an inadequate dose may be given, or the wrong diluent may be used. Animals may receive an incomplete vaccination series and failure may be because of a lack of recommended booster doses. Given the wide diversity of vaccine types, and especially the different properties of inactivated and modified live vaccines, it is critical that the manufacturer’s storage and handling instructions be completely followed. These may be product specific. Vaccinators must read and follow the manufacturer’s recommendations for each product regarding: storage temperature, exposure to light during storage, and shaking of the product to assure uniform vaccine suspension (Chapter 8). Many vaccines, especially modified live products, are very temperature sensitive. They may lose potency very rapidly if warmed. Thus it is essential that the cold chain be maintained all the way from the manufacturer and vendor to the veterinarian and administration to the patient. Failure to do this can result in a significant decline in vaccine efficacy, an increased rate of vaccine failures, and possibly an increased prevalence of postvaccinal adverse events. As described in Chapter 8, it is essential that vendors, veterinarians, and ranchers carefully manage their vaccine inventory and storage to ensure that they remain effective. Should the vaccine be exposed to temperatures outside the recommended range, or if it changes color or if it is exposed to ultraviolet radiation in sunlight, consult with the manufacturer before seeking to use it. The vaccine may contain the incorrect strain of organisms or the wrong (nonprotective) antigens. For many bacteria and viruses there may be many different strains, substrains, and variants that can cause disease. Immunity may, however, be strain specific. This appears to be a greater problem with killed/inactivated vaccines where nonspecific innate protective responses are less than in modified live vaccines. In vaccines directed against highly changeable viruses such as influenza, escape variants may lead to apparent vaccine failure. Examples of such strain-specific vaccines also include those directed against bovine campylobacter and porcine pleuropneumonia (Actinobacillus). It is unfortunately not uncommon that a vaccination schedule is not followed, especially in the case of vaccine shortages. Lapsed vaccination should not be confused with a breakdown in vaccine effectiveness. The persistence of memory cells after antibodies have disappeared in addition to their rapid reactivation after re-exposure to antigens implies that an immunization schedule should never be started all over again. It should continue where interrupted regardless of the duration of the lapse. The consequences of vaccination lapses depend on several factors. Lapses are of greater significance during a primary course of vaccination than in older animals with a history of receiving multiple doses of vaccine. They also depend on the nature of the vaccine used. Some antibacterial vaccines give relatively short-lasting immunity even under ideal circumstances. Thus failure to boost may well result in loss of protection. In general, modified live viral vaccines give the longest immunity. If multiple vaccinated animals in a group have lapsed vaccinations, then herd immunity may drop to unsafe levels. The significance of this will depend on the risk of exposure to infected animals. In the case of a vaccine shortage, veterinarians may have to find alternative, perhaps more expensive products. Every failure in the established vaccination schedule is a matter of concern for both the individual and the herd. Regardless of how long an animal is overdue for a vaccine, administration of a single does will reactivate the immune system and regenerate protective immunity. There is no necessity to begin a vaccine series all over again. Vaccine failures may be attributed to the animal being vaccinated. Preexisting infection is probably the most common cause of apparent vaccine failure. Alternatively, an animal may simply fail to mount an immune response. The immune response, being a biological process, never confers absolute protection and is never equal in all members of a vaccinated population. Because immunity is influenced by many genetic and environmental factors, the range of immune responses in a large random population of animals follows a normal distribution (Figs. 9.2 and9.3). This means that most animals respond to vaccines by mounting an average immune response, whereas a few will mount an excellent response, and a few will mount a poor immune response. These poor responders may not be protected against infection despite having received an effective vaccine. It is impossible to protect 100% of a large outbred population of animals by vaccination. The size of this unreactive portion of the population will vary between vaccines, and its significance will depend on the nature of the disease. For highly infectious diseases against which herd immunity is poor and in which infection is rapidly and efficiently transmitted, such as foot-and-mouth disease, the presence of even a few unprotected animals can permit the spread of disease and disrupt control programs. Likewise, problems can arise if the unprotected animals are individually important, such as companion animals. In contrast, for diseases that are inefficiently spread, such as rabies in dogs, 70% protection may be sufficient to effectively block disease transmission within a population and provide significant herd immunity (Box 9.1).
Failures in vaccination
Administration failure
Incorrect storage or expired vaccines
Wrong vaccine
Lapses in vaccination
Animal failure
Failure to respond
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
Veterian Key
Fastest Veterinary Medicine Insight Engine
