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Vaccine Reactions

By Sarah Lim, DVM


Vaccinations are an important part of your pet’s routine and preventive veterinary care. Puppies and kittens, like babies, have immature immune systems when they are born and vaccines play an important role in preventing infectious diseases to which pets would otherwise be susceptible.

While vaccines are generally well-tolerated, it is not unusual for some pets to have some mild side effects after receiving a vaccination. These signs may last for 12 to 24 hours and could include discomfort or soreness at the vaccination site, mild fever or malaise, decreased appetite or 1-2 episodes of vomiting or diarrhea. Mild sneezing can occur if your dog has received an intranasal vaccine. These signs are generally self-limiting but you should let your veterinarian know if they last for longer than 24 hours. Inflammation or swelling at the vaccination site can sometimes occur as well. If this occurs, a warm compress could be applied gently to the site for 10 to 15 minutes 2 to 3 times for a day or two, if your pet will tolerate it. Your veterinarian should be notified if your pet seems to be experiencing significant discomfort or if there is any persistent swelling at the vaccination site as additional treatments may be recommended.

Less commonly, more severe side effects might occur since the purpose of immunizations is to stimulate the immune system. A pet’s immune response can be highly individual and can overreact in some cases, causing an allergic reaction. These reactions can happen within minutes or up to hours after receiving a vaccination. Signs of an allergic reaction could include:

  • Facial swelling, especially swelling around the eyes and muzzle
  • Redness and hives (or “bumps”) on the body*
  • Itchiness*
  • Nausea and drooling, protracted vomiting and/or diarrhea
  • Difficulty breathing or breathing that becomes noisy
  • Collapse

* Cats are less likely to experience the swelling, itchiness, or hives than dogs.

These signs are indications of a more serious adverse reaction and could potentially be life-threatening. If you observe these signs, you should immediately bring your pet back to your veterinarian or the closest veterinary emergency hospital for prompt treatment. Treatment may include administration of medications such as an anti-histamine and/or corticosteroids. Other therapies and/or hospitalization may be recommended depending on severity of the reaction. If you are noting facial swelling or hives at home and have Benadryl (diphenhydramine hydrochloride) immediately available, a dose of 1 mg per pound of body weight could be given orally right away. You should still seek veterinary attention right after that, as the allergic reaction could progress despite the administration of an anti-histamine and may require additional treatment.

If there is any question as to whether your pet is having an allergic reaction, you should immediately contact your veterinarian or an emergency veterinary hospital if your veterinarian’s office is closed. Notify your veterinarian prior to or during a wellness visit if your pet has had an allergic reaction to a vaccine in the past. At minimum, it may be recommended to pre-treat your pet with Benadryl before the visit and to split up the vaccines or limit the number administered at one time. Antibody titers may be an option in place of some vaccinations and may be considered if your pet has had a severe reaction.

Fortunately, the benefits of vaccines outweigh the risks. It is always best to have a discussion with your veterinarian to determine which are recommended based on your pet’s lifestyle and risk of exposure, to keep your pets happy and healthy!

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