Vaccine adverse reactions

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In veterinary medicine, there is no requirement to report adverse events (known or suspected) following vaccination. Although strongly encouraged, formal reports from veterinarians to the U.S. Department of Agriculture (USDA) or the vaccine manufacturer are seldom filed.

Definition: The USDA considers a vaccine adverse event to be any undesirable side effect or unintended effect (including lack of desired result) associated with the administration of a licensed biological product (i.e., a vaccine). Adverse events include any reaction that could compromise the health of the dog or cat, including the apparent failure to immunize. NOTE: An adverse event includes any injury, toxicity, or sensitivity reaction associated with the use of a vaccine whether or not the event can be directly attributed to the vaccine. In other words, it is appropriate to report any known or suspected reaction associated with vaccination.

Vaccine adverse event categories

The actual prevalence of vaccine adverse events in veterinary medicine is unknown. The table that follows is not comprehensive, but it does represent a summary of known and suspected adverse event categories.



Transient injection-site reactions

Visible or palpable lumps caused by an abscess, granuloma, or seroma; injection-site pain, pruritus, local swelling.

Sustained injection-site reactions

Permanent hair loss (generally associated with ischemic vasculitis), discoloration of skin, focal necrosis of skin (also called rabies vaccine ischemic vasculitis), granuloma (postvaccination “lumps”).

Transient non-specific systemic effects

Lethargy, anorexia, fever, regional lymphadenopathy, nonlocalizable soreness/discomfort, diarrhea, vomiting, encephalitis, polyneuritis, arthritis, seizures, behavioral changes.

Allergic (hypersensitivity) and immune-mediated reactions

Type 1 (acute anaphylaxis): Angioedema (acute-onset swelling affecting the head and ears especially), urticaria (hives), collapse, acute-onset diarrhea, vomiting, dyspnea, systemic anaphylaxis (shock), and death.


Type 2 (cytotoxic): Immune-mediated hemolytic anemia, immune-mediated thrombocytopenia.


Type 3 (immune-complex): Cutaneous ischemic vasculopathy (often attributed to rabies vaccine) that can occur at the injection site or a distant location (“satellite lesions”) (such as the ear tips, foot pads, tail, and scrotum, undefined immune-mediated diseases [polyarthritis, glomerulonephritis]) and “Blue-Eye” (corneal endothelialitis associated with modified-live CAV-1 vaccines [CAV-1 vaccines are no longer available in the United States]).


Type 4 (delayed-type hypersensitivity): Less clearly described; associated with diminished cellular immunity and the release of pro-inflammatory cytokines. May be associated with post-vaccinal granuloma formation.

Failure to immunize

Interference from maternally derived antibody is considered the most common cause; administration of vaccine at a volume/dose less than that prescribed by the manufacturer; genetically predisposed "non-responder" or “low responder;” inactivation of vaccine antigen (e.g., allowing reconstituted CDV vaccine to stand at room temperature for more than 2 hr); mixing of incompatible vaccines in the same syringe


Malignant transformation of mesenchymal cells in susceptible patients.

Multi-systemic infectious/inflammatory disorder

Described in young Weimaraner dogs. The syndrome is not well characterized. May be linked to a poorly characterized immune deficiency in the breed.

Transient immune suppression

When combination vaccines containing modified-live CDV and CAV-1 or CAV-2, along with other vaccines, are first administered to puppies, transient suppression of cell-mediated immunity (not known to be clinically significant) may occur as early as 3 days postvaccination and can persist for 7 or more days.

Reactions caused by the incorrect or inappropriate administration of vaccine

In addition to injection-site abscesses, fatalities have been reported (rare) following subcutaneous administration of avirulent-live Bordetella bronchiseptica bacterin (intended for intranasal administration).

Reactions associated with residual virulence of attenuated vaccine

Post-vaccinal cough and/or sneezing associated with intranasal administration of attenuated vaccine (e.g., B. bronchiseptica + parainfluenza virus or feline B. bronchiseptica, feline herpesvirus-1 + calicivirus).

NOTE: This is not vaccine “reversion to virulence.”

Vaccine-induced interference with diagnostic tests

Examples include: false-positive PCR test results for parvovirus antigen in feces in dogs recently vaccinated with a MLV parvovirus vaccine; leptospirosis vaccination may cause false-positive test results with commercially available diagnostic tests that detect antibody.

Reversion of vaccine virus to a virulent pathogen

Although frequently discussed in the literature, true reversion to virulence (vaccine-induced clinical infection) is considered rare to non-existent following administration of currently licensed vaccines as long as the vaccines are used in the species for which they were licensed. The potential for reversion to virulence exists when using attenuated (MLV) canine/feline vaccine in a wild, hybrid, or exotic animal.


Reporting a known or suspected adverse event (reaction)

Vaccine Manufacture: Companies that manufacture vaccines maintain a technical services section that will accept and address adverse event reports from veterinarians who use their product(s). Veterinarians are encouraged to report adverse events to the manufacturer(s) prior to contacting the appropriate regulatory agency. Manufacturers are required to maintain files of any reported vaccine adverse event.

USDA’s Center for Veterinary Biologics (CVB): Subsequent to reporting a known or suspected vaccine adverse event to the manufacturer, veterinarians practicing within the United States may contact the USDA, APHIS CVB in one of the following ways:

  • Online: Adverse Event Electronic Report Form
  • By fax or mail: Download the pdf form (17kb) and FAX to (515) 337-6120 or by mail to the CVB.
  • By telephone: Adverse events may also be reported by calling the CVB at (800) 752-6255.

Canadian Food Inspection Agency (CFIA)’s Canadian Centre for Veterinary Biologics (CCVB): Veterinarians practicing within Canada should notify the CCVB of suspected adverse events related to veterinary biologics. This can be done by notifying CCVB directly or through the licensed vaccine manufacturer or importer.

Additional information:

These guidelines are supported by a generous educational grant from
Boehringer Ingelheim USA Inc., Merck Animal Health, and Zoetis.