Vaccination Concerns and Potential Side Effects

by Lorie Huston, DVM   

In the past, dogs were routinely vaccinated against just about anything we could think of, on the off chance that they might be exposed to that disease, in which case the vaccine should protect the dog.

Today, we recognize that vaccines can sometimes have a downside as well. 

There can be side effects with any medication and vaccines are no exception.

Anaphylactic/Allergic Reactions

By far the most common adverse reaction to vaccinations in dogs is an allergic reaction.
  • A normal reaction to vaccines includes mild lethargy and soreness. A mild fever may also occur and these symptoms are generally not serious.
  • Vaccine reactions can occur directly after the administration of a vaccine or within 48 hours later.
  • Hives, swelling of the face, muzzle and ears and sometimes nausea are symptoms seen with an allergic reaction.
  • In more severe cases, the dog may go into shock and may also die suddenly. Fortunately, this complication is extremely rare. 

Which Dogs are Most Likely to Have Adverse Reactions to Vaccines?

Puppies and young dogs are most likely to suffer adverse reactions to a vaccination. Small breeds of dogs are more likely to be affected than larger breeds.

Administering multiple vaccinations at the same time is more likely to elicit an adverse reaction. 

However, interestingly, multi-valent vaccines (vaccines which protect against more than one disease in the same vaccine) do not appear to cause an increase in the incidence of adverse reactions.

Immune-Mediated Disease Linked to Vaccines

At this time, the link between immune-mediated disease and vaccinations administered to dogs is unclear. However, there is some evidence that vaccinations can at least predispose a pet to develop an immune-mediated disease.

More research is needed in this area but there is some concern that vaccination could have a long-term effect on the immune system, making it important to minimize unnecessary vaccinations.

Vaccine Preservatives, Mercury and Thimerosal

Some vaccinations contain various preservatives, including mercury and thimerosal (a compound containing 49.6% mercury by weight).

Preservatives are added to vaccines in order to reduce the potential for growth of micro-organisms such as bacteria or fungi in the event of an accidental contamination of the vaccine vial. It should be noted that not all vaccines contain mercury or thimerosal, however.

In human medicine, there have been concerns raised about the safety of mercury and thimerosal in vaccinations.

It was believed that these substances may be linked to the development of the development of autism or another neurodevelopmental disease. However, studies performed to investigate this causal relationship have failed to prove there is a positive correlation, according to the FDA publication Questions About Vaccines. Similar studies in animals have also failed to show a link between adverse reactions in pets and mercury or thimerosal-containing vaccines to date.

This continues to be a controversial and widely debated topic, however.


Adjuvants are substances that are added to vaccines to increase the body’s response to the vaccine and lead to better protection with smaller quantities of the biological portion of the vaccine.

Adjuvants have been implicated in causing cancer in pets. 

This is another reason that unnecessary vaccination should be eliminated from the vaccination protocol for individual animals. Currently, some vaccinations are manufactured that are free of adjuvants and many veterinarians consider these safer than adjuvanted vaccines.

Why Administer Vaccines to Dogs and Cats with the Risks Involved?

Despite the risks involved with vaccinating dogs and cats, vaccines still provide documented benefits which in most cases outweigh any risks involved with their administration.

However, vaccines protocols for an individual animal must assess the relative risk for that animal and only vaccines which can claim the potential for more benefit than risk should be administered.

Articles by Dr. Huston:
Lyme Is Lame (Pun Intended)
The Ticking Bomb
Don't Let Heartworm Become A Heartbreak!
Summer Perils: Blue-green Algae
Your Dog And Leptospirosis
Canine Parvovirus
Canine Distemper Virus
Why Is My Dog So Itchy? Top 5 Causes Of Itching In Dogs

Related articles:
To Booster Or Not To Booster


  1. Hi Y'all,

    What a timely piece. We have our wellness check tomorrow morning and will be discussing what vaccines to give to a dog with numerous and severe allergies. Since he is an adult the vet suggested only giving him the required rabies 3 yr. Different vets seem to have different feelings about the necessity of titers.

    BrownDog's Momma

  2. Hi BrownDog's Momma. It's always a decision between the lesser of the evils. Is the risk of infection higher than the potential side effects of the vaccine. That is always the question.

    The beauty of the titers is that it can give you peace of mind knowing that your dog is protected (the three core vaccines). Of course the issue isn't as straightforward, but if the titers come back positive (protection is high enough) helps one sleep at night.

    It makes sense to limit the vaccinations for a dog with severe allergies.

  3. We stopped automatically giving the dogs their annual booster shots a few years back after 2 vets [one holistic and one "regular"] advised us on the pros and cons of doing it.

    Unfortunately, many boarding kennels here still require the C5 before accepting a dog as guest. We haven't been on holidays for a while [since Rufus became sicker], so that hasn't been an issue. But I can see future problems with this. Some [more enlightened?] kennels do accept a health check cert/vet note in lieu of the C5.

    We understand that kennel cough is quite common and ...just a cough, though many kennels remain adamant about the dog getting vaccinated against it. Any thoughts on this? At least we don't have rabies here :)

  4. Yes, many kennels are adamant about it here too and I appreciate why they would. We even had an outbreak of kennel cough in the dog park here a year back.

    I think it a case like that it is better to get a pet-sitting service instead.

  5. Personally I would never stop giving my dogs their yearly vaccines. Too many dogs have gotten sick and passed around diseases who haven't been vaccinated.

    All meds have side affects and having 5 vets say different things doesn't give me enough piece of mind to scale back or stop.

    Sorry, can't do it. :)

  6. Hi Sisko. Well, you need to do what gives you peace of mind.

    It was a conclusion of AAHA (American Animal Hospital Association) that protection from the core vaccines last minimum of 3 years.

    There are not too many vets disputing that fact, though there are some. Many argue that the protection lasts long past three years.

    I don't like taking anybody's word for things either, so what we did was that we ran a titer. That shows exactly how much protection there is. Jasmine's showed that she has plenty of protection.

    Not all vaccines last this long though, Leptospirosis vaccine, for example lasts a year only. It is, however, a non-core vaccine and whether or not you want to vaccinate depends on your dog's life style and exposure.


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