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The first time I met Kendall, a mini schnauzer mix, was for her 1-year vaccine appointment. Her owner, Lydia, my technician, was checking out at the front desk when Kendall started to poop and vomit uncontrollably on the floor. We took her to the treatment room and couldn’t get a blood pressure reading. Kendall was experiencing anaphylactic shock–a rare, but serious allergic reaction–presumably to pet vaccinations she had just received. I gave Kendall injections of epinephrine, steroids, Benedryl, and IV fluids, and she quickly recovered.

Allergic reactions to vaccines are scary but very rare. A lot of clients ask what to expect after a pet’s vaccinations and what to watch out for in case of a vaccine reaction. Here’s what you need to know about common side effects and more rare, but serious signs that could indicate an allergic reaction.

Common Side Effects

You know how you might feel a little bit under the weather after a flu shot or maybe a little sore in the arm after a tetanus booster? Pets may experience mild symptoms after vaccination too. These side effects usually go away after a day or two and may include:

  • Soreness
  • Lethargy or tiredness
  • Sneezing, mild coughing, or “snotty” nose
  • Loss of appetite

When we vaccinate, we introduce antigens into the body. Antigens are molecules that train the immune system to recognize and attack disease-causing viruses and bacteria. Symptoms such as mild soreness and lethargy are perfectly normal and mean that your pet’s immune system is responding appropriately to the vaccine.

Signs of an Allergic Reaction

Allergic reactions to pet vaccinations are rare but serious. They typically occur within minutes to an hour after the vaccination. An allergic vaccine reaction is a medical emergency. Call your vet immediately if your pet experiences any of the following after getting a shot:

  • Sudden vomiting and/or diarrhea
  • Facial swelling
  • Itchy, bumpy, red skin (hives)
  • Difficulty breathing (severe coughing or gasping)
  • Collapsing

Vaccinating After a Reaction

If your pet previously had a vaccine reaction, it doesn’t mean your pet shouldn’t be vaccinated. It just means that we need to take added precautions around future pet vaccinations. Kendall, for instance, now receives a Benadryl one hour before a vaccine. She hasn’t had another allergic reaction to a shot in three years.

The other thing I tell owners is that having an allergic reaction to a vaccine one time doesn’t necessarily predict the same thing will happen again. Vaccines are made up of an active component–or antigen–and an adjuvant. Adjuvants are substances that help to boost the immune system’s response to the antigen. When a pet has an allergic vaccine reaction, we often don’t know whether the antigen or adjuvant is causing the allergy. Different vaccine makers may use different adjuvants. Formulas for vaccines may change over time. It’s possible that your pet may show no symptoms of allergy with subsequent vaccines.

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