- Here there is development of the disease despite vaccination. Apparent breakdowns usually occur several weeks or more after vaccination but within the normal described period of immunity. Possible causes of apparent vaccine breakdown include:
- Faulty (non-potent) vaccine. Uncommon, but possible. Report suspected cases to the manufacturer and/or other appropriate body.
- Incorrect storage of vaccine. A likely cause of non-potency. Do not use vaccines after their recommended use-by date, and always store them according to the manufacturer's instructions. Live vaccines are particularly liable to lose potency if stored incorrectly.
- Incorrect administration of vaccine. Most frequently the result of the injection of an inadequate dose of vaccine (for example through the skin twice). Administration by the wrong route (for example intranasal vaccines given subcutaneously or killed systemic vaccines given oronasally) will also be ineffective at provoking protective immunity.
- Inhibition of vaccination by maternally derived antibody. A common cause of apparent vaccine failure in young animals. If the last vaccine dose is given before the puppy or kitten is 12 weeks old, or if the animal has particularly high MDA, vaccination may not provide active immunity and protection.
- Animal already infected. Vaccination rarely prevents the development of disease in an already infected animal or elimination of a carrier state. Vaccination of carriers of viruses such as feline calicivirus and felid herpesvirus-1, which can cause persistent infection, will not provoke elimination of infection and these animals may later shed virus and sometimes develop clinical disease.
- Intercurrent disease or immunosuppression. Infection with immunosuppressive agents, either at the time of vaccination or later, may prevent or decrease protective vaccine-derived immunity.
- Infection with different organisms or strains from those contained in the vaccine. Before a clinical disease can be ascribed to vaccine breakdown, the causative organism(s) must be identified. Cat 'flu and kennel cough, for example, are clinical descriptions of syndromes with multifactorial aetiology, and some causes of infectious upper respiratory disease in cats and dogs are not included in any vaccine. Similarly, not all cases of haemorrhagic gastroenteritis in dogs are caused by canine parvovirus.
- Overwhelming infection Vaccines generally provide reasonable protection against the development of disease, but not against infection. Thus, infection with very large doses of a pathogen, particularly if vaccine derived immunity is beginning to wane, will sometimes lead to clinical disease.
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