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Vaccine Reaction FAQ’s for Vet Techs

by Cathy Barnette - Aug 4, 2020 9:05:11 AM
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If you find yourself working in a small animal general practice as a new vet tech, you will need to be familiar with vaccine reactions. Fortunately, serious reactions are relatively uncommon. Client questions about vaccine reactions, however, arise on a regular basis! 

An understanding of vaccine reactions will help you react quickly when these reactions do occur and help you provide valuable information to your clients.

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What’s the difference between a vaccine reaction and a normal immune response?

The term “vaccine reaction” is typically used to refer to an abnormal inflammatory response to a vaccine. It’s important to note, however, that the normal immune response to vaccination often produces signs that owners may mistake for a vaccine reaction. 

Pets often act sore, lethargic, or a little bit “off” after vaccines. Just like in humans receiving a flu vaccine, vaccination stimulates many of the same immune pathways that are triggered when the body is fighting off an infection. Preparing your clients for these common, expected signs of an immune response will help decrease their anxiety and help them effectively determine whether or not their pet is truly having an abnormal vaccine reaction.

True vaccine reactions can be divided into two categories: local reactions and systemic hypersensitivity reactions. Pets with a local reaction should be seen by a veterinarian at the earliest convenience of the client and practice, while pets with a systemic hypersensitivity must be seen immediately, on an emergency basis. 

How are local vaccine reactions handled in small animal patients?

Some animals experience pain or soreness for a day or two after an injectable vaccine. This pain is usually caused by adjuvants or other ingredients in the vaccine, not the actual vaccine antigen.1 These vaccines rarely require a veterinary visit.

In some cases, swelling may develop at the vaccine site within one or two days of an injectable vaccine. While these localized swellings do not require treatment (unless there is evidence of infection or an abscess), they should be seen by the veterinarian so that the clinical appearance of the swelling  can be documented and monitored.1 

If a post-vaccine swelling meets certain criteria, a fine needle-aspirate should be performed. Criteria for fine-needle aspiration include:

  • Post-vaccine swellings that persist for over one month
  • Post-vaccine swellings that increase in size
  • Post-vaccine swellings that are over 2 cm in diameter1

These criteria, especially in cats, could suggest a possibility of vaccine-associated neoplasia. A fine needle aspirate is the best way to determine whether or not this is the case and determine whether additional therapy is needed.


What types of systemic reactions can occur in response to vaccines?

There are three types of hypersensitivity reactions that can occur after vaccination:

  • Type I Hypersensitivity: acute anaphylaxis
  • Type II Hypersensitivity: cytotoxic reaction (immune-mediated hemolytic anemia,  immune-mediated thrombocytopenia)
  • Type III Hypersensitivity: immune complex deposition (vasculitis)

When most veterinarians and veterinary staff think of the term “vaccine reaction,” we are thinking of Type I hypersensitivity reactions. Type II and Type III hypersensitivity reactions may occur, but are less common and can be difficult to definitively link to vaccination.

What are the signs of a Type I hypersensitivity reaction?

Type I hypersensitivity reactions in dogs typically present as urticaria and angioedema (swelling of the head and neck). Some dogs also develop redness and itching of the face and ears. In rare cases, dogs may experience acute shock and collapse. 

In cats, the signs often differ. While urticaria and angioedema may occur, they are less common in cats than dogs. Instead, gastrointestinal signs such as vomiting and diarrhea may predominate. 

These reactions typically begin within minutes to hours of vaccine administration. A Type I hypersensitivity reaction is a medical emergency that requires immediate treatment. Any client who calls to report signs consistent with a Type I hypersensitivity reaction should be instructed to return to the veterinary clinic (or seek care at the nearest emergency clinic) immediately. 

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How are Type I hypersensitivity reactions treated?

Many vaccine reactions are mild; in these cases, dexamethasone (0.2 mg/kg IV) and diphenhydramine (2 mg/kg IM) are often sufficient to stop a Type I hypersensitivity reaction. In more severe cases, epinephrine, oxygen, and intravenous fluids may be required. 

Regardless of the therapy that is used, the pet should be monitored to ensure that the reaction resolves with treatment. Additionally, oral antihistamines and corticosteroids should be sent home with the owner and continued for at least three days, to control late-phase reactions.

All vaccine reactions should be reported to the vaccine manufacturer and the USDA Center for Veterinary Biologics. The manufacturer and the USDA monitor vaccine reactions, looking for trends and ensuring that basic safety standards are met. While it can be challenging to find time to report these reactions during a busy day in practice, it’s essential to ensure that someone in the practice (typically the vet, vet tech, or practice manager) completes this task by the end of the day. 

Can a pet with a history of vaccine reactions safely receive future vaccines?

It depends. Once a pet has had a vaccine reaction, their record should be flagged according to the practice’s standards. (Every practice has a different system for noting vaccine reactors.) The pet’s history of vaccine reactions must be taken into account when administering future vaccines. 

In pets with a history of a mild reaction, a single dose of oral or injectable diphenhydramine is typically administered 15-30 minutes prior to future vaccines, although the true benefits of this practice are not documented.2 The veterinarian may also recommend discontinuing non-core vaccines, if the pet is determined to be at low risk (or if the owner can make lifestyle changes that would place the pet in a low-risk category). 

If a pet has had a severe anaphylactic vaccine reaction, future vaccines are often avoided. This is also the case in pets with a history of immune-mediated hemolytic anemia or immune-mediated thrombocytopenia. A rabies vaccine is often required by law, however, and a documented reaction is often insufficient to waive this legal requirement. Therefore, the client and veterinarian should work together to discuss the risks and benefits of rabies vaccination and arrive at a reasonable solution. 

What can be done to minimize the risk of vaccine reactions?

The overall risk of vaccine reactions is relatively low. A study of over one million dogs found that approximately 0.4% of vaccinated dogs have a vaccine reaction.3 A similar study in cats found a vaccine reaction incidence of approximately 0.5%.4 While the risk of a vaccine reaction is low, there are additional measures that can further decrease this risk.

First, it’s important to perform a thorough lifestyle assessment on each patient. Non-core vaccines should only be administered to pets with a lifestyle that places them at risk of infection. 

Second, splitting vaccines over the course of multiple visits (when possible) can reduce the likelihood of a vaccine reaction. In dogs, each additional vaccine administered during a vaccine visit may increase the risk of a vaccine reaction by 27% in small dogs (< 10 kg) and by 12% in larger dogs (>10 kg).3 Cats also have an increased likelihood of vaccine reactions when multiple vaccines are administered within a single visit.4 

When pets are due for multiple vaccines at once, ask owners if they would be okay with splitting the vaccines into two separate visits, at least two weeks apart. If that is not feasible, the risk of reaction is still relatively low, but splitting vaccines can further decrease that risk (especially in small dogs). 

The Take-Home Message

Fortunately, vaccine reactions are relatively rare in both dogs and cats. By taking simple steps to reduce that risk even further (when appropriate) and being prepared to treat vaccine reactions if they occur, you can maximize the benefits of vaccines while minimizing the risks to your patients. 


  1. Datz C. 2009. Adverse vaccination events: Separating fact from fiction. DVM360. Retrieved from:

  2. AAHA. 2017. Vaccine Adverse Reactions. Retrieved from:

  3. Moore GE, Guptill LF, Ward MP, et al. 2005. Adverse events diagnosed within three days of vaccine administration in dogs. Journal of the American Veterinary Medical Association, 227(7), 1102–1108.

  4. Moore GE, DeSantis-Kerr AC, Guptill LF, et al. 2007. Adverse events after vaccine administration in cats: 2,560 cases (2002-2005). Journal of the American Veterinary Medical Association, 231(1), 94–100.


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About Cathy Barnette

Cathy Barnette is a practicing small animal veterinarian, freelance writer, and contributor to XPrep Learning Solutions. She is passionate about both veterinary medicine and education, working to provide helpful information to veterinary teams and the general public. In her free time, she enjoys spending time in nature with her family and leading a Girl Scout troop.

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