Vaccinations are a big part of the veterinary care we get for our pets. Because of the creation of safe and reliable vaccines we can be confident that if we vaccinate our pets appropriately there is almost no reason that we should have to suffer from canine distemper, parvo, feline panleukepenia (also known as feline distemper), rabies, and many other diseases that used to cause severe illness and frequent death in many pets while we veterinarians could only stand helplessly by, offering supportive care and hoping for the best.
In spite of the huge benefit gained by the general population of pets, there are some individuals who have adverse reactions to vaccines. The immune system is a complicated thing, and sometimes it behaves unpredictably when stimulated by a vaccine. The most common, but usually least serious reactions happen very shortly after the vaccine. The more serious problems take some time to develop and they are extremely rare.
In dogs the most common vaccine reaction we see is a dramatically swollen face with puffed up eyelids, a fat muzzle, and sometimes the appearance of itchy hives all over the body. Less commonly we may get a single bout of vomiting and diarrhea. These things usually happen within a few minutes to hours after receiving a vaccine because some quick reacting immune system cells have been inadvertently triggered to cause inflammation in the area where they reside. These cells are found primarily in the skin, the gastrointestinal tract, and in the lungs. When people have this type of reaction it tends to generate frantic behavior from health care providers because it can be a warning sign that they are in danger of going into anaphylactic shock, having their lungs swell shut, and possibly dying. People have the highest proportion of inflammatory cells in their lungs, followed by skin and then digestive tract. Dogs, on the other hand, have most of their inflammatory cells in the skin, followed by digestive tract, and the least in the lungs. What this means for dogs is that even though a big puffy face catches our attention quickly, and is probably fairly uncomfortable, it rarely results in more than inconvenience. I see this sort of reaction about two to three times a year, out of thousands of patients vaccinated. Benadryl usually stops the reaction quickly and most dogs are none the worse for wear.
Cats have a higher proportion of reactive inflammatory cells in their gastrointestinal tract, so their rapid onset vaccine reaction tends to involve sudden profuse vomiting, and sometimes diarrhea. Because it is difficult to give oral medication to a vomiting cat it is usually best to bring her back in to the vet to get some injectable medication to stop the reaction.
Ferrets are vaccinated against rabies and canine distemper virus. Their rapid onset vaccine reactions tend to be more severe, often starting with vomiting, but progressing to shock and collapse. Affected ferrets need treatment immediately, but most do quite well if treated quickly. Newer vaccines for ferrets are decreasing the incidence and severity of these types of reactions.
On very rare occasions an individual can develop dangerous or even fatal problems as a result of vaccines. Some animals can have their immune system decide to start destroying their own red blood cells or they can develop cancer at the injection site. It is impossible to predict who might have these problems, and it is terrible when it happens, but when the odds of dying from a vaccine reaction are truly miniscule, and the odds of your dog dying from parvo if not vaccinated are huge, the math comes out in favor of vaccinating.