Vaccine reactions are a common concern in small animal medicine. Fortunately, true vaccine reactions are uncommon. Client concerns about vaccine reactions, however, are a very common issue! 

Having a thorough understanding of vaccine reactions is important, so that you can address client concerns and handle these reactions quickly when they occur.

Is it really a vaccine reaction?

Owners often report that their pet is uncomfortable, lethargic, or a little bit “off” after vaccines. In many cases, clients refer to these changes as a vaccine reaction. While they’re technically correct (these signs do occur in response to vaccination!), such subtle signs do not indicate a true hypersensitivity reaction. 

Lethargy and even a mild fever are common following vaccination. Vaccines trigger an immune response, which can lead pets to feel a little bit crummy for a day or two, just like they would if having an immune response to an infectious organism. It’s no different than when we humans get a flu vaccine. Lethargy and/or a mild fever do not indicate a need for medical treatment and do not suggest a need for special precautions with future vaccines. 

Local Reactions 

Local reactions, such as pain and swelling, are not uncommon after vaccines. Pain may develop immediately after a vaccine and is usually caused by adjuvants or other ingredients in the vaccine, not the actual antigen.¹ Swelling may develop within one or two days and may persist for up to a week. This swelling is often a reaction to the vaccine antigen and should be documented.¹ No treatment is indicated, however, unless there is evidence of infection or an abscess.

A fine needle aspirate should be performed on post-vaccine swellings that meet any of the following criteria:

• Swellings that persist for over one month
• Swellings that increase in size
• Swellings that are over 2 cm in diameter¹

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

Hypersensitivity Reactions 

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

• Type I Hypersensitivity: Acute anaphylaxis. These reactions are characterized by urticaria, angioedema (especially of the head and neck), shock, and death. In cats, urticaria and angioedema may be less apparent; gastrointestinal signs such as vomiting and diarrhea may predominate. 
• Type II Hypersensitivity: Cytotoxic reaction. Immune-mediated hemolytic anemia and immune-mediated thrombocytopenia may be associated with vaccination in some pets.
• Type III Hypersensitivity: Immune complex deposition. Localized cutaneous vasculitis has been associated with some rabies vaccines and ear-tip vasculitis may also be associated with vaccination. 
  
  

When most veterinarians and veterinary staff think of the term “vaccine reaction,” we are thinking primarily of Type I hypersensitivity reactions. 

Vaccine reactions may occur minutes to hours after the administration of vaccines and require immediate treatment. In mild reactions, dexamethasone (0.2 mg/kg IV) and diphenhydramine (2 mg/kg IM) should be administered. In more severe reactions, 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 is resolving with treatment. Additionally, antihistamines and corticosteroids should be continued for at least three days, to control late-phase reactions.¹ 

It is important to report all vaccine reactions to both 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 that you do so (or delegate this task to a trusted vet tech). 

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 will need to 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.² If a pet has had a severe vaccine reaction, it is often recommended to avoid that vaccine in the future. This can present a challenge in cases of a rabies vaccine reaction, however, because rabies vaccination is often required by law. 

Minimizing the Risk of Vaccine Reactions

There are two factors that have been shown to be correlated with the risk of vaccine reaction: patient size (in dogs) and the number of vaccines given at once. 

A study of over one million dogs found an overall incidence of 38.2 vaccine reactions for every 10,000 vaccinated dogs.³ In this study, each additional vaccine administered during a vaccine visit increased the risk of a vaccine reaction by 27% in small dogs (< 10 kg) and by 12% in larger dogs (>10 kg).³ Based on these statistics, it is obvious that dogs that are due for multiple vaccines at one time would benefit from having these vaccines divided over multiple visits. While splitting vaccines in this manner benefits all dogs, it is especially beneficial in smaller breed dogs.

A similar study of nearly half a million cats found a similar risk of reactions, at 51.6 reactions per 10,000 vaccinated cats.⁴ Like dogs, cats also demonstrated an increased likelihood of reactions when multiple vaccines were administered within a single visit. 

When pets are due for multiple vaccines at once, ask owners if they would be okay with returning for multiple visits in order to separate vaccines. If that is not feasible and multiple vaccines must be administered at a single visit, the risk of reaction is still relatively low. However, splitting vaccines into two visits (at least two weeks apart) will decrease that low risk even further. 

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. 

References

1. Datz C. 2009. Adverse vaccination events: Separating fact from fiction. DVM360. Retrieved from: https://www.dvm360.com/view/adverse-vaccination-events-separating-fact-fiction-proceedings

2. Ford RB. 2011. Postvaccinal adverse events. DVM360. Retrieved from: https://www.dvm360.com/view/postvaccinal-adverse-events-proceedings

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.