Recently, I wrote about claims made by a veterinary homeopath that booster vaccination was unnecessary because single, initial vaccination provides lifelong immunity for our pets. There is ample evidence that this is frequently untrue, but today I ran across an article that illustrates quite nicely that this claim is false.
Ippei Watanabe, Kentaro Yamada, Akira Aso, Okio Suda, Takashi Matsumoto, Takaaki Yahiro, Kamruddin Ahmed, and Akira Nishizono. Relationship between Virus Neutralizing Antibody Levels and the Number of Rabies Vaccinations: a Prospective Study of Dogs in Japan. Jpn. J. Infect. Dis., 66, 17-21, 2013
This study involved sampling blood from 756 dogs and evaluating the rabies antibody levels. This antibody level, and the proportion of dogs with enough antibody to be protected from rabies, was then compared to the number of rabies vaccinations the dogs had received, the age and sex of the dog, the time since the last rabies vaccine, and other factors. The results illustrate the need for a rational, evidence-based approach to determining the number and frequency of vaccinations.
The authors found that puppies who had never been vaccinated for rabies almost never had protective antibody levels despite receiving some antibodies through nursing.
we found that only 1 unvaccinated dog exhibited protective VNA levels whereas many unvaccinated dogs exhibited inadequate VNA levels…we set the VNA level of 0.25 IU/ml as the cutoff value…Of the 72 unvaccinated puppies aged 90 days or less, 11 samples (15.3z) exhibited VNA levels of Æ0.25 IU/ml. In contrast, only 1 of the 35 unvaccinated puppies aged over 90 days (2.9z) exhibited VNA levels of Æ0.25 IU/ml.
Of the dogs vaccinated only once, between 33% and 77% exhibited protective antibody levels at various time points during the 13 months after vaccination. Of the five time points sampled, only at one (13 months) did the proportion of protected dogs reach a level above the 70% believed to be necessary to prevent rabies from becoming established in the population at large. When the dogs who are not vaccinated are considered, it is unlikely that 70% of the population will ever be protected if only one rabies vaccine is given to vaccinated dogs.
Of the dogs vaccinated more than once, the proportion protected ranged from 79% to 100%. However, of the 7 time points sampled, the proportion of dogs protected was greater than 97% at all but one (25 months after vaccination). This indicates that giving at least two vaccinations to each dog ensures that nearly all dogs will be protected for at least two years. Since the rabies vaccine has elsewhere been demonstrated to protect the vast majority of dogs for at least three years, this is a reasonable minimum frequency for vaccination. However, better data is needed to determine how often we should be recommending revaccination since there is some evidence adequate population protection may last quite a bit longer than three years.
Interestingly, there did not appear to be any difference in the proportion of the population with protective antibodies between dogs receiving two vaccinations and dogs receiving more than two, so this suggests that two vaccines per dog may be sufficient to provide herd immunity. However, the specific details of how long protection lasts in what percentage of dogs aren’t clearly known, and this information is needed to make reliable decisions about limiting rabies vaccination to two vaccines only.
Because a high antibody titer is indicative of protection from disease for rabies (which is not true for all other diseases), individual dogs could have their titers measured to confirm they are protected and don’t require repeat vaccination. However, a low titer does not necessarily mean the dog is susceptible to the disease, so it isn’t necessarily a good indication of when a vaccine is needed. And, of course, the majority of the long-term health risks attributed to vaccination by vaccine opponents like Dr. Falconer have not actually been shown to be cause by vaccines, so choosing not to vaccinate based on these purported risks is not a rational, evidence-based decision.
This study, of course, does not provide all the answers about how often and when to vaccinate even for rabies, much less all the answers about all vaccines. It illustrates, however, both the kind of information that has to be accumulated to make rational decisions about vaccination and the nonsense of Dr. Falconer’s claims that one vaccine can be assumed to provide lifelong immunity for most dogs. As always, reality is complex and nuanced, with inevitable uncertainty, but we still make better decisions for our dogs if we accept this and try to work with the evidence we have rather than making simplistic and mistaken generalizations about medical procedures like vaccination.