There are many reasons that a vaccine may fail to confer protective immunity on an animal. Here we discussed briefly.
In many cases vaccine failure is due to unsatisfactory administration. For example, a live vaccine may have died as a result of poor storage, the use of antibiotics in conjunction with live bacterial vaccines, the use of chemicals to sterilize the syringe, or the excessive use of alcohol when swabbing the skin. Sometimes animals given vaccines by nonconventional routes may not be protected. When large flocks of poultry or mink are to be vaccinated, it is common to administer the vaccine either as an aerosol or in drinking water. If the aerosol is not evenly distributed throughout a building, or if some animals do not drink, they may receive insufficient vaccine. Animals that subsequently develop disease may be interpreted as cases of vaccine failure.
Failure to Respond
Occasionally, a vaccine may actually be ineffective. The method of production may have destroyed the protective epitopes, or there may simply be insufficient antigen in the vaccine. Problems of this type are uncommon and can generally be avoided by using only vaccines from reputable manufacturers. More commonly, 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. Since the immune response is influenced by a large number of genetic and environmental factors, the range of immune responses in a large random population of animals tends to follow a normal distribution. This means that most animals respond to antigens by mounting an average immune response, whereas a few will mount an excellent response, and a small proportion will mount a poor immune response. This group of poor responders may not be protected against infection despite having received an effective vaccine. Therefore, it is essentially impossible to protect 100% of a random 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.
Thus, 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 unprotected animals could permit the spread of disease and would thus 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, 70% protection may be sufficient to effectively block disease transmission within a population and may therefore be quite satisfactory from a community health viewpoint. Another type of vaccine failure occurs when the normal immune response is suppressed. For example, heavily parasitized or malnourished animals may be immunosuppressed and should not be vaccinated. Some virus infections induce profound immunosuppression. Animals with a major illness or high fever should not normally be vaccinated unless for a compelling reason. Stress may reduce a normal immune response, probably because of increased steroid production; examples of such stress include pregnancy, fatigue, malnutrition, and extremes of cold and heat.
Correct Administration and Response
Even animals given an adequate dose of an effective vaccine may fail to be protected. If the vaccinated animal was incubating the disease before inoculation, the vaccine may be given too late to affect the course of the disease. Alternatively, the vaccine may contain the wrong strain of organisms or the wrong (nonprotective) antigens.
1Dixit K. Parasana, 2I. H. Kalyani, 3P. M. Makwana1Ph. D. Scholar, 2Professor and Head, 3Assistant Professor,
Department of Veterinary Microbiology,
College of Veterinary Science & Animal Husbandry, Navsari