Since the original companion animal vaccines were developed and licensed more than 50 years ago, there has been a continuing
effort to make vaccines safer and more efficacious. Today, it is generally agreed that the vaccines licensed for dogs and
cats have not only saved millions of lives but also have an excellent safety record. Despite widespread use of vaccines today,
documentation of post-vaccinal adverse reactions ('events') in veterinary medicine is limited.
In veterinary medicine, post-vaccinal adverse event reporting is "passive"…which means that data collection is limited to
those reports voluntarily submitted by veterinarians to the manufacturer and/or the USDA. For a variety of reasons, there
is currently no national database of veterinary vaccine adverse events accessible to the profession. In large, awareness
of vaccine reactions is limited to personal experience, anecdotal reports, and a few published studies.
This presentation is intended to characterize various types of vaccine adverse events in companion animals, provide information
on how to report known/suspected adverse events, and offer suggestions for mitigating the risk of vaccination in patients
with a prior vaccine adverse event history.
Defining an adverse event
According to the USDA, a vaccine adverse event can be defined as any undesirable side effect or unintended effect (including
lack of desired result) associated with the administration of a licensed biological product (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 negative event
associated with vaccination.
HOWEVER……vaccines are biological products and, as such, can be expected to provoke minor reactions lasting from a few hours
to as long as a few days. Rarely do these self-limiting side effects escalate into serious adverse events. For this reason,
veterinarians are encouraged to inform clientele that their pet, regardless of breed/size, may manifest transient side effects
for up to 2, and possibly 3, days following administration of any vaccine or any combination of vaccines. Side-effects commonly
observed include: reduced appetite, loss of appetite (lasting for 1 or 2 feedings), pain at the injection side, lethargy
(lack of activity), reluctance to walk/run (discomfort?), and mild fever. Treatment is not generally necessary; however,
some veterinarians do report administering short-term treatment (e.g., a non-steroidal anti-inflammatory drug) given either
before or at the time of vaccination to mitigate expected minor reactions. It is recommended that clientele be advised to
contact the practice in the event any physical/behavioral manifestations progressively worsen or continue beyond 2 to 3 days.
Clientele should be advised to contact the practice at any time if signs of systemic illness, such as vomiting, diarrhea,
seizures, collapse, facial swelling, or difficulty breathing, develop.
Types of vaccine adverse events
The actual prevalence of vaccine adverse events in veterinary medicine is unknown. The list that follows is not considered
to be comprehensive but does represent a summary of known and suspected event categories.
- Injection-site reactions: (minor) lumps caused by abscess, granuloma, or seroma, pain, swelling, (potentially severe)
hair loss associated with ischemic vasculitis.
- Transient post-vaccinal non-specific illness: lethargy, anorexia, fever, regional lymphadenomegaly, soreness, abortion,
encephalitis, polyneuritis, arthritis, seizures, behavioral changes, hair loss or color change at the injection site, respiratory
- Allergic (hypersensitivity) and immune-mediated reactions:
• Type 1 (acute anaphylaxis): angioedema (especially the head), anaphylaxis (shock), and death.
• Type 2 (cytotoxic): immune-mediated hemolytic anemia; immune-mediated thrombocytopenia (suspected only; causality
has not been confirmed)
• Type 3 (immune-complex): cutaneous ischemic vasculopathy associated with rabies vaccine that can occur at the
injection-site or a distant location (eg, ear tips); undefined immune-mediated diseases.
- Failure to immunize: maternal antibody interference with vaccination is considered the most common cause; administration
of vaccine at a volume/dose less than that prescribed by the manufacturer; "non-responder" (genetic predisposition?); inactivation
of vaccine antigen (eg, allowing reconstituted CDV vaccine to stand at room temperature for more than 2 hours), mixing of
incompatible vaccines in the same syringe.
- Tumorigenesis: vaccine-associated sarcoma (especially in cats) or other tumors.
- Multi-systemic infectious/inflammatory disorder of young Weimaraner dogs: may be linked to a poorly characterized
immune deficiency in the breed.
- Vaccine-induced immune suppression: associated with attenuated (MLV) vaccine. Rarely may lead to development of
severe disease associated with opportunistic infection.
- Immunosuppression: When combination vaccines containing MLV CDV and CAV-1 or CAV-2, along with other vaccines,
are first administered to puppies, immune-suppression occurs beginning 3 days after vaccination, and can persist for 7 or
- Reactions caused by the incorrect or inappropriate administration of vaccine: fatalities have been reported following
subcutaneous administration of an avirulent-live Bordetella bronchiseptica bacterin (intended for intranasal administration);
inadvertent or intentional administration of vaccine by the intravenous route.
- Reactions associated with residual virulence attenuated vaccine: post-vaccinal sneezing associated with intranasal
administration of attenuated vaccine (eg, B. bronchiseptica + parainfluenza virus)
- Vaccine-induced interference with diagnostic tests: false-positive PCR test results for parvovirus antigen in feces
in dogs recently receiving a MLV parvovirus vaccine.
- Reversion of vaccine virus to a virulent pathogen: generally considered rare to non-existent among currently licensed
canine vaccines when vaccines are used in the species for which they were licensed. This can become a significant problem
when vaccine licensed for use in dogs/cats is used in wild and/or exotic animals.