(When the cat was apparently successfully vaccinated but disease occurs within the normal recommended period of immunity.)

Possible causes

• Assuming the vaccine was potent and had been stored and given correctly, other possible causes include the following:
• Although protection from the current vaccines is reasonable, even under ideal circumstances, protection is not necessarily complete in all cats.
• Intercurrent disease, e.g. with FeLV or FIV, either at the time of vaccination or thereafter. There is evidence from experimental studies that previous infection with FIV renders subsequent vaccination with an inactivated FCV vaccine significantly less effective in protecting against FCV challenge.
• Overwhelming infection. Studies have shown that the majority of vaccine breakdowns occur approximately 6 months after vaccination, and it may be that as immunity declines, a high challenge dose could overcome lowered levels.
• Maternally derived antibody may have interfered with initial vaccination programme. The duration of maternal immunity in kittens can be variable and there is little information on the interaction between this and vaccination.
• Other agents not incorporated in the vaccine may be causing disease, e.g. feline C. psittaci, B. bronchiseptica.
• Although most vaccines incorporate strains of FCV that are reasonably cross-reactive (e.g. F9), they do not appear to protect equally well against all strains of FCV. The vaccines may therefore not provide complete protection against some of the viruses isolated from vaccine breakdowns. For the FHV component this is not such a problem, since there is only one antigenic strain, which seems relatively stable.
• Carriers: the common occurrence of carriers ensures there is a ready supply of virus to infect susceptible cats. Vaccination does not eliminate infection from cats that are already carriers, and a previously unexposed cat systemically vaccinated can become a field virus carrier if later exposed to field virus. Intranasally vaccinated cats may become carriers of the attenuated FCV vaccine component, but there is some evidence that vaccination may protect in the short term against cats becoming FHV field virus carriers.

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