Initial Series

Because maternal antibodies passively protect newborn animals, it is not usually possible to successfully vaccinate animals very early in life. If stimulation of immunity is deemed necessary at this stage, the mother may be vaccinated during the later stages of pregnancy, the vaccinations being timed so that peak antibody levels are achieved at the time of colostrum formation. Once an animal is born, successful active immunization is effective only after passive immunity has waned. Since it is impossible to predict the exact time of loss of maternal immunity, the initial vaccination series will generally require administration of multiple doses. Current guidelines for essential canine and feline vaccines, for example, indicate that the first dose of vaccine should be administered at 8 weeks of age, followed by a second dose 3 to 4 weeks later, and concluding at about 15 weeks of age. (These are not, strictly speaking, booster doses. They are simply designed to trigger a primary response as soon as possible after maternal immunity has waned.) All animals should then receive a booster dose 12 months later. Administration of vaccines to young animals is discussed in Chapter 21.

The timing of initial vaccinations may also be determined by the disease. Some diseases are seasonal, and vaccines may be given before disease outbreaks are expected. Examples of these include the vaccine against the lungworm Dictyocaulus viviparus given in early summer just before the anticipated lungworm season, the vaccine against anthrax given in spring, and the vaccine against Clostridium chauvoei given to sheep before turning them out to pasture. Bluetongue of lambs is spread by midges (Culicoides variipennis) and is thus a disease of midsummer and early fall. Vaccination in spring will therefore protect lambs during the susceptible period.

Revaccination and Duration of Immunity

As pointed out in Chapter 18, the phenomenon of immunological memory is not well understood; yet it is the persistence of memory cells, B cells, plasma cells, and T cells after vaccination that provides an animal with long-term protection. The presence of long-lived plasma cells is associated with persistent antibody production so that a vaccinated animal may have antibodies in its bloodstream for many years after exposure to a vaccine. It is believed that these long-lived plasma cells are stimulated to survive by activation with microbial PAMPs acting through TLRs and that it is the antibodies that are mainly responsible for long-term protection.

Revaccination schedules depend on the duration of effective protection (Table 24-1). This in turn depends on specific antigen content, whether the vaccine consists of living or dead organisms, and its route of administration. In the past, relatively poor vaccines may have required frequent administration, perhaps as often as every 6 months, to maintain an acceptable level of immunity. Newer, modern vaccines usually produce a long-lasting protection, especially in companion animals; many require revaccination only every 3 years, whereas for others, immunity may persist for an animal’s lifetime. Even killed viral vaccines may protect individual animals against disease for many years. Unfortunately, the minimal duration of immunity has, until recently, rarely been measured, and reliable figures are not available for many vaccines. Likewise, although serum antibodies can be monitored in vaccinated animals, tests have not been standardized, and there is no consensus regarding the interpretation of these antibody titers. Even animals that lack detectable antibodies may well have significant resistance to disease. Nor is there much information available regarding long-term immunity on mucosal surfaces. In general, immunity against feline panleukopenia, canine distemper, canine parvovirus, and canine adenovirus is considered to be relatively long-lasting (>5 years). On the other hand, immunity to feline rhinotracheitis, feline calicivirus, and Chlamydophila is believed to be relatively short. One problem in making these statements is variability among individual animals and among different types of vaccine. Thus recombinant canine distemper vaccines may induce immunity of much shorter duration than conventional, modified live vaccines. There may be a great difference between the shortest and longest duration of immunological memory within a group of animals. Duration of immunity studies are confounded by the fact that in many cases older animals show increased innate resistance. Different vaccines within a category may differ significantly in their composition, and although all vaccines may induce immunity in the short term, it cannot be assumed that all confer long-term immunity. Manufacturers use different master seeds and different methods of antigen preparation. The level of immunity required for most of these diseases is unknown. A significant difference exists between the minimal level of immunity required to protect most animals and the level of immunity required to ensure protection of all animals.

Annual revaccination has been the rule for most animal vaccines since this approach is administratively simple and has the advantage of ensuring that an animal is regularly seen by a veterinarian. It is clear, however, that vaccines such as those against canine distemper or feline herpesvirus induce protective immunity that can last for many years and that annual revaccination using these vaccines is unnecessary. A growing body of evidence now indicates that most modified live viral (MLV) vaccines induce lifelong sterile immunity in dogs and cats. In contrast, immunity to bacteria is of much shorter duration and often may prevent disease but not infection. Old dogs and cats rarely die from vaccine-preventable disease, especially if they have been vaccinated as adults. Young animals, in contrast, may die from such diseases, especially if not vaccinated or if vaccinated at an incorrect age. A veterinarian should always assess the relative risks and benefits to an animal in determining the use of any vaccine and its frequency of administration. It may therefore be good practice to use serum antibody assays such as ELISAs, if available, to provide guidance on revaccination intervals. Persistent antibody titers may indicate protection, but this is not guaranteed, especially if cell-mediated immune mechanisms are important for protection. Likewise, animals with low or undetectable serum antibody levels may still be protected as a result of persistence of memory B and T cells capable of responding rapidly to reinfection.

Notwithstanding the previous discussion, animal owners should be made aware that protection against an infectious disease can only be maintained reliably when vaccines are used in accordance with the protocol approved by the vaccine-licensing authorities. The duration of immunity claimed by a vaccine manufacturer is the minimum duration of immunity that is supported by the data available at the time the vaccine license is approved. This must always be taken into account when discussing revaccination protocols with an owner.