From the Guidelines: Vaccination reactions

Since the first vaccine was invented in 1796, vaccinations against preventable diseases have enhanced global health and life expectancy in humans and other animals. Yet some clients may mistakenly not trust that vaccines offer protection—for them or their pets—and conclude that vaccination carries unacceptable risk.

To help veterinarians ease clients’ concerns about vaccinating their dogs, these highlights from the newly updated 2022 AAHA Canine Vaccination Guidelines (2024 Update) summarize potential undesirable postvaccination events, and list strategies to keep their likelihood low. Review the full guidelines and related updates to refresh or reconfirm your 2025 canine vaccination protocols.

Types of postvaccination complications

Licensed canine vaccines provide protective immunity in the vast majority of vaccinated dogs and pose a low risk of serious postvaccination problems. Still, it’s helpful to recall why unexpected consequences may occur.

Vaccination that fails to provide protective immunity may stem from:

• An inadequate immune response in a vaccinated patient
• Infection exposure before full vaccination
• Maternally derived antibodies interference in pediatric puppies
• Waning immunity from age-related immune system dysfunction (immunosenescence) in geriatric dogs
• Improper vaccine storage, handling, or administration
• Manufacturer errors that reduce product potency (e.g., instability, expiration, or improper storage)

Along these lines, the guidelines note that:

• There’s no definitive evidence of immune nonresponders to a standard initial vaccination series in specific breeds of puppies.
• Geriatric dogs may have diminished naïve T-cell ability to contest new antigens, but they don’t lack memory cells for antigens encountered in early or mid-life.
• There’s no evidence that concurrently giving multiple antigens or vaccines produces ineffective protective immunity.

Adverse reactions associated with vaccination may occur:

• With inappropriate administration of a modified-live product
• Secondary to innate immune responses to the vaccine
• With specific cell-mediated or humoral immune responses to vaccine components
• If vaccine antigens return to virulence (unlikely for licensed vaccines)

Although type I hypersensitivity reactions have been linked with vaccination, vaccines have been less consistently associated with other conditions that can be immune-mediated. This suggests that factors other than vaccine antigens may provoke immune disease after vaccination.

Genetics can influence both beneficial immune responses and adverse reactions, but phenotype (e.g., breed) only partially indicates genotype. So increased vaccine reaction risk in some breeds more likely stems from some family lines that produce genetically predisposed individuals. Explain to clients that postvaccination adverse events risk pertains to individuals.

In genetically predisposed dogs, the labeled (pathogen) antigens in canine vaccines typically don’t provoke undesirable immune responses. The antigens that trigger IgE reactivity may stem from cell culture media (used in vaccine virus propagation) that contains xenogeneic proteins such as fetal bovine serum. Therefore, combination multi-pathogen vaccines don’t carry more adverse events risk than single-component vaccines. For example, single-component rabies vaccines have more diverse proteins related to virus propagation than combination vaccines that contain other viruses propagated similarly, or than other single-component vaccines.

Leptospirosis vaccination news

An update to the guidelines reflects that vaccination against leptospirosis—a life-threatening, endemic, and zoonotic disease—now has core status. Leptospirosis vaccination is recommended for dogs ≥12 weeks of age and of any breed, sex, or lifestyle who go outside (including kennels or daycares) in rural, suburban, and urban environments in wide-ranging geographic areas and climates (arid to tropical) in North America.

Canine leptospiral postvaccination complications were a historical concern, but reformulated vaccines are now less likely to induce problems. With <53 adverse events per 10,000 doses, adverse reactions to leptospiral vaccines are rare, and most are minor—injection site pain, mild fever, anorexia, and malaise—as can occur with other vaccines. Adverse event odds after leptospirosis vaccination were less than those after rabies vaccination or combination distemper, canine adenovirus type 2, parvovirus, and parainfluenza vaccination. Serious anaphylaxis has been reported in dogs given leptospiral vaccines no more frequently than for dogs given other vaccine antigens.

Reducing the risk of postvaccination problems

The guidelines offer strategies to mitigate postvaccination complications and adverse events risk.

• Help clients book visits to ensure dogs complete their primary vaccine schedules and receive proper interval boosters.
• Follow manufacturers’ vaccine storage and handling instructions and administration recommendations (e.g., giving a vaccine after it reaches room temperature).
• Reduce antigenic stimuli by giving fewer vaccines during a single visit and allow at least two weeks between vaccination visits. This approach is especially helpful in small dogs. (“Split dosing” to reduce vaccine volume given is not labeled or advised and can result in professional liability.)
• Because smaller and younger dogs are more likely to have adverse reactions of any type, give the initial leptospirosis vaccination to puppies ≥12 weeks of age and give the second dose two to four weeks later, which aligns with the last two puppy vaccine visits.
• Give mucosal or intranasal vaccines when possible (in accord with guidelines and manufacturer labels).
• For dogs with a vaccine reaction history, give diphenhydramine before vaccination and limit the number of vaccines administered. (Pretreatment is generally not recommended in an at-risk breed without previous reactions.)
• Have clients sign an informed consent document for patients with existing medical conditions or health concerns who receive vaccines.
• Be well prepared to treat vaccine-associated anaphylaxis—a rare, but life-threatening event—to ensure the best outcome possible.
• Report possible vaccination adverse events to the vaccine manufacturer or the USDA, or both.
• Avoid client vaccination miscommunication or mixed messages by educating team members about which vaccines are recommended and why.

Photo credit: FatCamera via E+ Getty Images 

Disclaimer: Trends content is meant to inform, educate, and inspire by providing an array of diverse viewpoints. Any content published should not be viewed as an official stance, position, or endorsement by the American Animal Hospital Association (AAHA) or its Board of Directors.