Vaccine overview and types

Vaccines are one of the medical and public health successes of the 19th and 20th centuries. Their use has reduced morbidity and mortality more than any other intervention in human and veterinary medicine. Vaccination of companion animals protects the health of the individual animal, improves animal welfare in community settings (e.g., shelters), protects public health (e.g., rabies and leptospirosis), and reduces the occurrence of infectious diseases that transmit mainly within a species (e.g., canine variants of rabies virus, canine distemper, and canine parvovirus). Vaccines have mitigated the impact of infectious diseases on populations through herd immunity so successfully that some dog owners may hold the perception that vaccination is no longer necessary. Although individual dogs with low-risk lifestyles (i.e., minimal exposure to other animals) may benefit from herd immunity, unvaccinated individuals are still more vulnerable to infection, and reductions in population-level vaccination rates without eradication of the pathogen will inevitably result in a recurrence of disease at outbreak levels. This has been clearly demonstrated by recurrent canine distemper and parvovirus outbreaks in shelters, and by recent outbreaks of measles in human populations where reduced vaccine coverage exists.

Vaccine efficacy, assessed during product development, is measured as the proportionate reduction of disease in vaccinated groups compared with unvaccinated groups. Although necessary for the purposes of licensing, vaccine efficacy calculated under these controlled settings may not equate to the population impact of the vaccine in real-world settings. This impact, known as vaccine effectiveness, is more difficult to quantify, especially in veterinary medicine, which lacks the robust surveillance systems for monitoring the numbers of individuals vaccinated and disease cases. Vaccination failures, namely, the occurrence of disease in an animal that has received an appropriately administered vaccine against that disease, are rare but should be expected because no vaccine achieves 100% effectiveness.

Vaccination failures can occur for many reasons including:

• Failure of the vaccinated patient to mount an adequate immune response.
• Exposure to the infection before being fully vaccinated.
• Interference of maternal antibodies.
• Improper storage or handling of the vaccine, including inappropriate administration.
• Waning immunity (e.g., immunosenescence, or age-related deterioration of the immune system).
• Vaccine manufacturing errors, such as lack of potency due to instability, expiration, or improper storage.

Vaccination failures should be promptly reported to the manufacturer. These reports are essential for detecting changes in product performance due to defects in particular lots of vaccine. In the United States, if a veterinarian is unable to report to the manufacturer, reports can be made directly to the United States Department of Agriculture (USDA) Center for Veterinary Biologics. More information and instructions on reporting are available online from the USDA here.

Although no vaccine produces complete immunity or protection, the term duration of immunity (DOI) is commonly used for the length of time a vaccine is expected to produce a robust immune response and protection against illness following exposure. DOI data, unlike vaccine efficacy (the reduction in disease in vaccinated animals compared with unvaccinated animals) data, are not required for licensure by the USDA. Exceptions to this include rabies vaccines and, recently, new vaccines for which no pre-existing products are available. Vaccine labels historically recommend booster doses every year. Increasingly, data are available from postlicensing studies demonstrating that the effect of many vaccines persists for extended periods. In some cases, DOI data have been submitted to the USDA to update vaccine labels. These data have also been considered in vaccine guidelines developed by various stakeholder groups. Because data also reveal differences in serologic titers following administration of different vaccine formulations for the same pathogen, extrapolation about efficacy and DOI between products may not always be appropriate.^1–11

Table 1 lists the characteristics of the four general categories of canine vaccines based on the physical attributes of the vaccine immunizing antigen.