Failures in Vaccination

There are many reasons why vaccination may fail. In some cases, the vaccine may not be effective because it contains strains of organisms or antigens that are different from the disease-producing agent. In other cases, the method of manufacture may have destroyed the protective epitopes, or there may simply be insufficient antigen. Such problems are relatively uncommon and generally can be avoided by using vaccines from reputable manufacturers. An effective vaccine may fail due to unsatisfactory administration. For example, a live vaccine may be inactivated as a result of improper storage, use of antibiotics in conjunction with a live bacterial vaccine, chemical sterilization of syringes, or excessive use of alcohol on the skin. Administration by nonconventional routes may also affect efficacy.  When vaccine is administered to poultry or mink by aerosol or in drinking water, the aerosol may not be evenly distributed throughout a building, or some animals may not drink adequate amounts. Also, chlorinated water may inactivate vaccines. If an animal is incubating the disease before vaccination, the vaccine may not be protective; vaccination against an already contracted disease is usually impossible.

The immune response, being a biologic process, never confers absolute protection nor is equal in all individuals of a vaccinated population. Because the response is influenced by many factors, the range in a random population tends to follow a normal distribution: the response will be average in most animals, excellent in a few, and poor in a few. Those with a poor response may not be protected by an effective vaccine; it is difficult to protect 100% of a random population by vaccination. The size of this unresponsive population varies among vaccines, and its significance depends on the nature of the disease.  For highly infectious diseases in which herd immunity is poor and infection is rapidly and efficiently transmitted (eg, foot-and-mouth disease), the presence of unprotected animals can permit the spread of disease and disrupt control programs. Problems also can arise if the unprotected animals are individually important, as in the case of companion animals or breeding stock. In contrast, for diseases that are inefficiently spread (eg, rabies), 60-70% protection in a population may be sufficient to effectively block disease transmission within that population and therefore may be satisfactory from a public health perspective.

The most important cause of vaccine failure in young animals is the inability of an antigen to impart immunologic memory whether or not passive maternal antibodies are present. Vaccines also can fail when the immune response is suppressed, eg, in heavily parasitized or malnourished animals (such animals should not be vaccinated). Stress, including pregnancy, extremes of cold and heat, and fatigue or malnourishment, may reduce a normal immune response, probably due to increased glucocorticoid production.