There’s a common misconception out there about vaccines that small animals should have lower “doses” of a vaccine than larger animals. This is a natural assumption stemming, most likely, from our familiarity with how medicines are dosed. However, while there are some differences in the amount of a vaccine given in different species, it is far less of a difference than would be expected if vaccines worked like drugs.
An analogy I often use to explain this is that drugs work like a dimmer switch for a lamp. You get some light with a small movement of the switch. The farther you push the switch, the brighter the light gets. There is a minimum movement below which the light stays off and a maximum above which the light won’t get any brighter. In between, however, the amount of light is proportional to the amount you move the switch. This is much like a drug, which often will have more effects the larger the dose given (though, of course, the reality is much more complicated since different drugs work in different ways, and in the real world pharmacokinetics and pharmacodynamics are highly complex and variable).
Since the dose of the drug is really the concentration in the blood or tissues, the larger an animal, the more drug has to be given to get the same amount in each milliliter of blood or each gram of body tissue. Assuming the simplest kind of drug dosing relationship, the size of the animal is directly proportional to the dose needed, meaning a dog that is 10kg will need twice as much drug as a dog that weighs 5kg. Again, reality is much more complicated, but as a basic starting point, we often dose drugs in milligrams per kilogram of body weight to reflect this kind of relationship.
However, vaccines don’t work this way. A better analogy for vaccines is a traditional light switch rather than a dimmer switch. There is a critical point at which the light goes from all off to all on. Movement below the threshold yields no light, and additional movement above the threshold gives no more light. For vaccines, there is a minimum immunizing dose (MID), a threshold at which the immune response is fully stimulated. Vaccines don’t work by being distributed throughout the body at a certain concentration, like drugs, but by triggering an essentially all-or-nothing immune response through interacting with special cells in the immune system.
The MID does vary a little by size, but not much. A horse, for example, will get complete protection form rabies after receiving only about twice as much vaccine as a dog, though the dog may be much less than half the size of the horse.
The same logic applies to any adverse effects from drugs and vaccines. In general, harmful effects get worse as the tissue or blood concentration of a drug goes up, which is very closely related to the dose given. With vaccines, adverse effects are unpredictable and not clearly related to dose. It is true that smaller dogs are more prone to allergic reactions to vaccines than larger breeds, but this likely has to do with genetic differences between breeds, not size. Smaller individuals within a breed have not been shown to be more prone to such reactions than larger individuals. The difference in size between members of the same species is almost never great enough to affect the MID, and the idea that a 10lb dog needs half the vaccine of a 50lb dog simply isn’t consistent with the biology of vaccines and the immune system.
Nevertheless, alternative medicine advocates frequently recommend smaller “doses” of vaccine for smaller dogs. There is not yet any real evidence to indicate that this would be an effective strategy to maintain immunity while reducing the risk of adverse effects. A recent “study” by Dr. Jean Dodds claims to provide some such evidence. However, this research has been supported by the American Holistic Veterinary Medical Foundation (AHVMF), and it bears many of the characteristic features of that group’s approach to research. As I’ve pointed out before, the AHVMF and associated groups and individuals seem to feel the purpose of scientific research is not to uncover the truth but to convince others of claims they already “know” are true based on personal experience or simple belief. Dr. Dodds herself has certainly shown this to be her approach before.
While Dr. Dodds’ study is only a pilot trial, and thus isn’t intended to prove anything, it is already being used as if it were evidence for giving smaller doses of vaccine to smaller dogs. The study has been published in the AHVMA journal, which is not accessible except to AHVMA members. From the information reported on her web site, it isn’t possible to evaluate completely what, if anything, we can reasonably conclude from it. But it is clear even from this limited information that the study does not provide a reason to change vaccination practices.
Thirteen dogs under 12lbs who had not been vaccinated in at least three years were enrolled in the study. Over a two-year period, antibody titers for distemper and parvovirus were taken for each dog at enrollment and then 4 months and 6 months after vaccination with a parvo/distemper vaccine. The vaccine was given as a “half dose,” though exactly what this means or how it was determined isn’t clear. The results reported were only that antibody levels went up in all of the dogs after vaccination and stayed higher than initial levels through the 6-month check.
This tells us almost nothing of relevance to the question of whether lower doses of vaccine can protect small dogs and reduce their risk of adverse events. Is the Hemopet titer a validated titer test with meaningful cutoff levels? What were the original titer levels? Were they already protective? How much did they increase, and would this make a difference between immunity and susceptibility? What vaccine history did the dogs have? Were any even susceptible to these diseases and, if so, would the vague half dose have protected them? Did any have adverse reactions? Had they had any adverse reactions to full-dose vaccination? Would they be less likely to have such reactions at the lower dose?
The unanswered questions are nearly endless, and many of them are crucial to the actual question. While a pilot trial, again, is only intended to test whether a real study is feasible and safe, the reality is that this trial is already being used to imply that giving less of a vaccine to smaller dogs is safer and just as effective as giving the intended dose. Dr. Dodds implies that on her site, and others reporting her results elsewhere do the same. Nothing about this trial justifies that claim.
Hopefully, more rigorous and transparent research will be done, but I am not optimistic. The goal is clearly to convince others of something Dr. Dodds and most of the AHVMA members already believe on faith, not to find out if this belief is actually true. As is often said about politicians, alternative medicine proponents often use science the way a drunk person uses a streetlamp: for support, not for illumination.