Active immunization, achieved through proper vaccination, plays a critical role in the control of infectious diseases, both for individual cats and for the cat population as a whole. Some vaccines also reduce the potential for spread of zoonotic infections to humans (e.g., rabies). The benefits of routine, widespread vaccination are clear: the incidence of serious disease caused by pathogenic organisms, such as feline panleukopenia virus (FPV), can be reduced dramatically when widespread vaccination is practiced. However, the quality of vaccine-induced immunity is influenced by the patient’s environment, the characteristics of the vaccine, the pathogen, and the patient’s immune competence. Accurate prediction of the outcome of vaccination or the likelihood of exposure to a pathogen is impossible. Therefore, it is important that veterinarians inform cat owners that vaccination is not a guarantee of protection.
In general, kittens are more susceptible to infection and disease than adults. Thus, they represent a primary target population for immunization. As part of a routine wellness program, the vaccination needs of all cats should be assessed annually, in conjunction with a comprehensive physical examination, modifying vaccination and other control recommendations as necessary based on the current risk (see “Vaccination Risk-Benefit Assessment”).
Kittens born to immune queens lack significant transplacentally acquired antibodies4 and instead absorb specific maternally derived antibodies (MDA) through colostrum, which provides important protection during early life. Most absorption occurs within 24 hours of birth. However, this MDA also interferes with active immunization. Serum MDA inhibits immunoglobulin (Ig)G production within the neonate through negative feedback mechanisms. It also neutralizes vaccine antigens and prevents them from stimulating an immune response. MDA then declines at a variable rate. Maternally derived IgG in kittens in one study was lowest at around 3–4 weeks of age, and serum IgG and IgA increased dramatically at 5–7 weeks of age.4 These results suggested that kittens may be susceptible to infectious diseases at about 1 month of age, perhaps as much as 2 weeks earlier than puppies.
Nevertheless, it is critical to recognize that there is considerable individual variation in the rate of decline of MDA, and some kittens maintain high concentrations for months.5 The persistence of MDA is one of the most common reasons for vaccine failure. The amount of MDA in a kitten at any one time point cannot be predicted because it varies depending on the titer of the dam and the amount of colostrum ingested after birth. As a result, a series of vaccinations is administered to kittens every 2–4 weeks through 16–18 weeks of age in order to increase the chance that successful immunization will occur soon after the decline of MDA to sufficiently low titers. The series is started no sooner than 4 weeks of age, because neonates are more likely to develop vaccine organism–associated disease and may not respond well to vaccination. During administration of the series, a window exists when MDA concentrations are high enough to interfere with immunization but are not sufficient to prevent natural infection. This window of susceptibility can be minimized by decreasing the interval between vaccinations in the series, although use of intervals less than 2 weeks can interfere with successful immunization, especially with attenuated live vaccines.
Once vaccination has been successfully achieved after the decline of MDA, it is generally recommended that a booster vaccine be given 3–4 weeks later (this is especially important for inactivated vaccines, although a boostering effect will also occur following revaccination with attenuated live vaccines). This means that the series must be extended 3–4 weeks beyond the period in which the decline in MDA occurs, with the final vaccination dose being a booster. In the past, it was recommended that revaccination be performed 1 year after the initial kitten series, and then for most vaccines every 3 years thereafter. However, owing to studies that suggest up to onethird of kittens may fail to respond to a final core vaccine at 16 weeks and may have blocking MDA at 20 weeks, the WSAVA recommends that the 1 year vaccine (FPV + FHV-1 + FPV only) be replaced with revaccination at 6 months of age.2,6,7
The risk of infection and disease varies with factors such as the age and health of the cat, magnitude of exposure to the infectious agent, the pathogenicity of the agent, and the vaccination history of the cat. Some of the factors that impact an individual animal’s ability to respond to vaccination include interference from MDA, congenital or acquired immunodeficiency, concurrent disease, inadequate nutrition, chronic stress, and very young or old age. Some vaccines (e.g., those for FPV) induce a stronger protective response than others (e.g., those for FHV-1). Because vaccine-induced protection is variable andnot absolute, vaccination should not be used as the only form of protection, and other control measures, such as those that reduce exposure to infectious agents, should also be employed.
Potential Therapeutic Benefits of Vaccination
Active immunization can enhance non-specific immunity, leading to reduction in disease caused by non-target pathogens. One study showed that vaccination of cats with an intranasal FHV-1-FCV vaccine was associated with reduction in clinical signs following challenge with B bronchiseptica.24 More studies are needed to assess the non-target effects of different vaccine types. There is also interest in whether vaccines might provide therapeutic benefits in cats already infected with target pathogens. Improvement in chronic upper respiratory tract signs that were previously refractory to other treatments was documented in 13 cats vaccinated with an intranasal FHV-1-FCV vaccine.35 Most vaccines, however, provide no therapeutic benefit, as clearly documented for FeLV vaccines.36