Allergic Reactions to Pet Vaccines

Allergic Reactions to Pet Vaccines

Quick Facts from a Veterinarian

Jennifer Hawkins, DVM

How Do Vaccines Work?

Vaccines are parts of viruses or infectious bacteria that are known to stimulate an immune response in your pet. After the puppy or kitten booster series is complete, the immune system has a “memory” of these virus or bacteria parts and thus will mount a swift response when the animal is challenged with the disease later on.

Many pets feel tired for a day or so after receiving a vaccination. This is because the immune system is working hard to make those antibodies and memory cells for the future.

Some pets will have a little bit of soreness at the injection site. Local pain or soreness and lethargy for approximately 24 hours are very normal responses to vaccination.

What are Signs of Allergic Reaction to Vaccination?


While pets infrequently suffer from an allergic reaction to vaccination, it’s important for pet owners to be aware that if an allergic reaction takes place, it can happen immediately or up to 48 hours after vaccines have been administered.

Usually, if an allergic reaction is to occur, it is not with the very first vaccine but rather with boosters. That is because, again, the immune system must have a memory of this disease in order to generate a response.

An allergic reaction is an aberrant response to the vaccination or, rather, an over-reaction to the vaccine. Signs can be vomiting and/or diarrhea within the first few hours after administration of the vaccine. Puffiness may be noted, especially around the eyes, muzzle and ears.

Another potential symptom is hives, or bumps, all over the body. If any of these signs are noted, or if at any time you are concerned with how your pet is acting after vaccination, you should call your veterinarian immediately. Very rarely, anaphylactic reactions can occur in which a pet may collapse or even die moments after vaccination. Again, this is extremely rare.

How is an Allergic Reaction Diagnosed?

Typically, no formal tests are needed. The veterinarian’s exam findings combined with the knowledge of very recent vaccination are enough to diagnose a vaccine reaction.

How is an Allergic Reaction Treated?

Your veterinarian will determine the best approach for your pet. Mild or moderate cases may be treated with an injectable antihistamine and/or a cortisone injection whereas more severe reactions may also require epinephrine and aggressive intravenous fluid therapy and support.

What’s the Prognosis for a Dog or Cat Suffering From a Vaccine Reaction?

The prognosis is good. It is extremely rare for an animal to die or suffer long term effects from a vaccine reaction.

Are Some Vaccines More Likely to Cause Reactions Than Others?

In the past, the Leptospirosis vaccine component present in some canine distemper combination vaccines was implicated in causing increased vaccine reactions. Recently, new information suggests that this may no longer be true, however, many veterinarians still avoid this vaccine component.

In some cases, cats may have a localized inflammatory reaction and develop a subsequent sarcoma due to FeIV or Rabies vaccine. While this is not classified as an allergic reaction, it should also be monitored and treated should a lesion occur at the injection site. Talk to your veterinarian about what’s right for your pet.

Are Some Pets More Likely to Experience Vaccine Reactions than Others?


While there are no particular breed predispositions, it has been found that the greater the number of vaccines administered at one time, the greater the risk of vaccine reaction. This is especially true for small dogs or cats. Large dogs seem to be able to tolerate receiving multiple vaccines at once whereas small dogs appear to have an increase in the chance of vaccine reaction.

To help avoid this, often vaccines are administered at different times, perhaps a week apart, rather than all at once. Some have suggested that half the normal dose of the vaccine may be more appropriate for small animals; however, this is not true. While it may seem odd that a small dog and a large dog are both to receive the same volume of vaccine, the immune system doesn’t see things that way.

A good analogy is that of one with a peanut allergy — ingesting half a peanut rather than the whole peanut is not less likely to cause a reaction.

Should Pets Still Receive Future Vaccines if a Vaccine Reaction Has Occurred in the Past?

Your veterinarian will determine if your animal is still a candidate for vaccines. Typically, your veterinarian will have notations in the record noting that your pet is reacts atypically to vaccines to avoid future reactions.

In some cases, a vaccine component might be avoided in the future or your pet may be pre-treated with anti-inflammatory drugs prior to vaccination. To maintain compliance with local and state rabies ordinances, you may still be required to have your pet vaccinated against rabies virus unless a rabies exemption is permitted by the public health officer in your area.

Remember: An allergic reaction to vaccination is not common. It’s always wise to be not any changes in your pet’s behavior after any vaccinations have been administered. If you have any concerns prior to vaccination, discuss with your veterinarian. Based on your location and your pet’s lifestyle, your veterinarian will determine the appropriate vaccine protocol for your pets.

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Dr. Hawkins received her veterinary degree from the University of California, Davis and practices in Orange County, Calif., where she also works at local animal shelters. In addition, she is an advisory faculty member for aspiring veterinary technologists at California State Polytechnic University Pomona. Dr. Hawkins is an active member of the Southern California Veterinary Medical Association where she served as president on the Board of Trustees.