At last, a smoking gun ... discovered pointing directly at Chihuahuas, Dachshunds, Maltese, Yorkies and other small dogs ... in fact, pointed at all dogs receiving multiple vaccines during one clinic visit.

Many scientific studies and taskforce reports have altered my view of vaccination over the years, but none have stunned me as much as "Adverse events diagnosed within three days of vaccine administration in dogs" by Drs. Moore, Guptill, Ward, et al. This two-year study of vaccine reactions (from data gathered at 360 Banfield clinics in 2002 and 2003) concluded: "Young adult small-breed neutered dogs that received multiple vaccines per office visit were at greatest risk of a VAAE [Vaccine Associated Adverse Event] within 72 hours after vaccination." And that's not all the report revealed.

In the study (published in JAVMA, the Journal of American Veterinary Medical Association in October, 2005), 1.2 million dogs received almost 3.5 million vaccine doses. Reactions reported within 3 days (as designated by computer codes) included nonspecific vaccine reactions, allergic reactions, urticaria (hives), anaphylaxis (severe, whole-body allergic reaction), cardiac arrest, cardiovascular shock and sudden death. For each 10,000 dogs vaccinating, 38 adverse reactions were reported.

You're probably thinking: just 38 reactions per 10,000 dogs? That's not too bad. But bear in mind that this study did NOT include:

• Reactions reported more than 72 hours after vaccination (thus eliminating reactions taking longer to develop or be discovered, such as injection site cancers, autoimmune diseases, skin diseases and other major conditions);
• Reactions that occurred but were never reported by clients;
• Conditions not recognized by the vet as vaccine reactions;
• Conditions not selected for this study (seizures weren't on the list, nor were countless other common reactions);
• Conditions not recorded by the vet. The 2007 World Small Animal Veterinary Association (WSAVA) Vaccine Guidelines reports "gross under-reporting of vaccine-associated adverse events...";
• Reactions in dogs also getting a heartworm shot, presumably because of the increased risk of reaction (currently, vets are warned not to give Proheart 6 with a vaccination).

The study's researchers (6 of 7 were vets) recommended that veterinarians should communicate the increased risk of multiple vaccines to clients before obtaining their consent to vaccinate. At this writing, a full four years after the study's publication, I suspect few clients have actually been warned about the risk of multiples vaccines at one visit or about special risks to smaller dogs. Were you ever warned?

Factors Increasing the Risk of the Vaccine Adverse Reaction

Body Weight. The reaction rate increased significantly as body weight decreased. That is, small dogs were at greatest risk for a reaction. Risk for dogs weighing 11 pounds or less was four times greater than the risk for dogs weighing 99+ pounds. Medium-sized dogs also had increased risk over larger dogs.

For all vaccines and for the rabies vaccine given alone, the reaction rate for dogs weighing 22.2 to 99 lbs. was approximately half the rate of dogs weighing less than 22.0 lbs. Little dogs had 32+ reactions per 10,000; medium-sized dogs, 15+; large dogs, none.

Neutering/Spaying. Neutered dogs had a 27% to 38% greater risk versus sexually intact dogs.

Age. Dogs 1.5 to 2.5 years of age had a 35% to 64% greater risk of reactions (with rates increasing up to two years and declining thereafter) than puppies 2 to 9 months old. The risk was least for dogs six years of age and older.

Number of vaccines per office visit. The risk significantly increased as the number of vaccines given at each visit increased. In little dogs (under 10 lbs.) each dose increased risks by 27%; in dogs weighing more, each dose increased risk by 12%.

Taking all dogs into consideration, each additional vaccine given at each office visit increased the rate of vaccine reaction by 24.2%. All three dogs in the study with recorded deaths had each received four or more vaccines at their last office visit.

Three or more vaccines given at once increase the risk of a vaccine reaction 50% over the risk of a single shot. Giving five simultaneous vaccines doubles the risk!

Breed. Among breeds with 5,000 or more dogs vaccinated during the study period, the most vaccine reactions per 10,000 dogs were found, in order: in Dachshunds, Pugs, Boston Terriers, Miniature Pinschers and Chihuahuas. Next came Maltese, Miniature Schnausers, Jack Russells, Toy Poodles and Yorkshire Terriers. Mid-size dogs (like Lhasa Apsos, Bichons and Beagles) followed. At the bottom of the list was Chow Chows, German Shepherds and Rottweilers.

Purebred Status. The vaccination reaction rate for mixed-breed dogs was in the bottom fifth of all rates. The researchers state: "safety trials that use such dogs may underestimate the reaction rates that would occur in purebred dogs."

Why Does a Dog's Weight Have Such a Big Impact?

The researchers report: "Vaccines, in contrast to virtually all veterinary pharmaceuticals, are prescribed on a one-dose-fits-all basis, rather than by body weight."

I have always been shocked that a Chihuahua puppy and an adult Great Dane are given the same dose shot: 1 ml. They get the same volume of virus or bacteria plus the same volume of adjuvants (boosting agents like aluminum), preservatives (like mercury), antibiotics, stabilizers and foreign tissue cultures (like fetal calf serum). All these ingredients are known to cause vaccine reactions. (Learn more about vaccine ingredients at the CDC.)

The study's researchers go on to say that during a vaccine's pre-licensing trial, manufacturers investigate the safety of excessive doses of vaccines "but only in a limited number of dogs. The results of this study suggest that trials in dogs that weigh [22 lbs.] underestimate the expected VAAE rate in smaller dogs."

The risk of a vaccine reaction in this study population was inversely related to a dog's weight. This weight/response relationship was also suggested by a study in which toy breeds had significantly more reactions than other dogs, although body weight was not evaluated.

How Do You Avoid Reactions to Vaccines?

The study detailed here reports the problems, but not the remedy. They only recommend that veterinarians advise clients of the risks.

Regrettably, I have been unable to find you a link to the study on-line. Your vet may have on-line access if he/she subscribes to JAVMA (J Am Vet Med Assoc. 2005 Oct 1;227(7):1102-8). You can read a short summary or have your non-subscribing vet request the article for a small fee at http://www.ncbi.nlm.nih.gov/pubmed/16220670 or http://avmajournals.avma.org/doi/abs/10.2460/javma.2005.227.1102?journalCode=javma.

Note: A smaller study for cats entitled "Adverse events after vaccine administration in cats" turned up similar results to the dog study.

If your vet gives multiple shots in a visit, you should insist that he/she read this study. If your vet has already read it, he/she should explain to you why you weren't informed of the risks to your dog of multiple shots, especially if your dog was small or medium sized.

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