Back to Stories & News

Updated September 25, 2025 — Peanut, a 5-pound Chihuahua deeply loved by Morris Animal Foundation staff, had a scare after receiving a routine leptospirosis booster. The unexpected reaction prompted her veterinarian-owner to rethink her vaccine schedule. After some thoughtful adjustments, Peanut was reported to be doing well. While uncommon, cases like Peanut’s raise important questions for pet owners and veterinarians alike, fueling ongoing conversations about vaccine safety, necessity, and individual risk.

Clarity is crucial as vaccine hesitancy grows among pet owners, compounded by recent disease outbreaks and news stories. In this article, we delve into understanding vaccination mechanisms and scrutinize data on adverse events. By examining success stories and shortcomings, we aim to empower pet owners to make informed decisions about their furry friends' vaccinations.  

The Basics of Vaccination: Getting the Immune System Ready to React   
Our pets encounter potential disease-causing agents daily, relying on their immune system's prowess for defense. Vaccination capitalizes on the immune system, aiming to train it to efficiently recognize and combat foreign or abnormal agents.  Vaccine strategies are designed to prompt a quicker immune response, lessen disease severity, or prevent symptoms altogether.  

Vaccines vary in composition, using live attenuated viruses, killed agents, or novel technologies like mRNA vaccines. The formulation also includes mediums that aid immune response, known as adjuvants. The length of vaccine protection, influenced by factors like virus mutability, dictates the need for boosters.  

The medium in which vaccines are delivered is another essential component. While it can be an inert substance, in some instances, these other substances can also stimulate the immune system, generating a more robust immune response. These substances are commonly referred to as adjuvants – more on that later.  

Vaccines can stimulate the immune system in different ways. Some vaccines seldom require a booster, while others offer relatively short-term protection. An example is the Bordetella vaccine given to dogs every six months to prevent kennel cough. The duration of immunity often dictates when vaccines and boosters are recommended, with most vaccines falling somewhere in between.  

There are a few known factors and many unknown reasons why there is a difference in how long a pet is protected. One factor that influences the duration of immunity is how fast a virus mutates. The more quickly a virus mutates, the more tweaking is required to keep the vaccine effective. Influenza is a classic example of a rapidly changing virus requiring yearly vaccine updates — unfortunately, a lot of the why isn’t known yet.    

Examining Adverse Reactions   
Several studies have confirmed that adverse reactions to vaccination are low in cats and dogs. A 2023 article published in the Journal of the American Veterinary Medical Association describes results from a study that looked at adverse events associated with dog vaccinations. This study repeated work by the same research group in 2003. The team wanted to learn if anything had changed since the publication of their first study when it came to adverse events.    

The team reviewed the data from about 4.6 million dogs. They looked at adverse reactions reported within three days of vaccination. They found a rate of 19.4 adverse reactions per 10,000 vaccinations. Younger dogs (2-18 months) were more likely to react than older dogs (but the numbers were still low, roughly 25 per 10,000 vaccinations).  

The smaller the dog by body weight, the higher the risk for an adverse reaction. The team also found that some breeds were more likely to have a vaccine reaction than others. The top three breeds with higher adverse events include:  

  • French bulldogs    
  • Dachshunds  
  • Boston terriers

However, the rates of adverse events remained low for these breeds (roughly 45 to 55 events per 10,000 vaccinations).  

The team also confirmed previous data that suggested that the greater the number of vaccinations given during a visit, the higher the risk of an adverse reaction. Owners need to understand this refers to the actual number of vaccines, not the number of antigens in a vaccine (for example, the combination distemper/parvovirus/adenovirus/Parainfluenza virus vaccine would count as one vaccine).  

The same team published a study in 2007 looking at adverse events in 500,000 cats within 30 days of vaccination (note this time frame was quite a bit larger than the two dog studies). The team reported that the rate of adverse events was 51.6/10,000 cats vaccinated. Like dogs, the more vaccines given at one time, the higher the likelihood of a reaction. Neutered cats also had a higher risk of a vaccination reaction, and the chance was most significant for cats that were approximately 1 year old. Lethargy was the most commonly reported sign, followed by swelling, pain, or soreness at the injection site.  

Unlike in the dog study, the team followed cats for one to two years. The extended follow-up period focused on monitoring for injection-site sarcoma, a serious cancer that can develop at previous injection sites. In this study, the team did not find that cats who had localized vaccine reactions developed ISS during the study follow-up period.  

The bottom line for the dog and the cat study is that the rate of adverse reactions is still meager.    

Success Stories and Ongoing Research: Impactful Vaccination Endeavors    
In 1986, two kittens showed up at the home of a pair of animal lovers who were reeling from the loss of their last cat to cancer. One was pure white with startling blue eyes, coined Sinatra, and the other was a red tabby that reminded the owners of a beloved cat from their childhood – Little Red.    

The kittens seemed healthy and happy and promptly wormed their way into their new owners’ hearts. They received the vaccines available at the time and were spayed and neutered. Everything seemed on track for a long and healthy life.  

However, one day, the new owners noted that Sinatra was breathing harder than usual, and Little Red was beginning to lose weight. Devastation struck when they discovered that both were infected with the feline leukemia virus. Little Red succumbed to leukemia within three weeks, and Sinatra followed a month later, succumbing to lymphoma, a type of cancer, in his chest. It was a crushing blow to people who prided themselves in caring for their pets.  

In 1986, a vaccine for feline leukemia virus had just entered the market. It was known to have side effects – fever, lethargy, inappetence. However, as vaccine adoption became more widespread and the vaccine improved, veterinarians noted something unique – the rate of certain cancers, especially those in young cats, plummeted. Further study revealed the reason: the use of vaccines against FeLV.  

Talk to any veterinary cancer researcher about cancer in cats, and they often speak of the “pre-FeLV vaccine time” and the “after times.” FeLV infection was a significant driver for the development of lymphoma and other blood cancers in cats. The feline leukemia vaccine's effectiveness in reducing this cancer has been astounding. Before FeLV vaccination became widespread, the average age of lymphoma diagnosis in cats was 3 to 5 years. In the era following the introduction of the feline leukemia vaccine, the incidence of FeLV infection has decreased significantly.  

Of course, cats still get all types of cancer, but the remarkable success of the FeLV vaccine continues to spur research looking for other cancer-causing viruses in cats and other species, too.    

Injection-site Sarcoma and Vaccination: A Failure, A Risk Factor or Something Else?   
Dr. Rachael Thomas, a research scholar at North Carolina State University and Foundation-funded scientist, didn’t set out to study veterinary cancer. But a childhood encounter with the disease left an indelible mark on her.  

“As a child, I had a cat that developed an injection-site sarcoma,” Thomas said. “Bodkin was our family cat, and I formed a close bond with her as the youngest child. We grew up together. I remember that when I heard she had cancer, I was old enough to understand it was serious. I was anxious and paralyzed by the fact there was nothing we could do. I felt vulnerable.”  

Tumor formation can result from injections in susceptible individuals, causing injection-site sarcomas. The cruelty of ISSs is they arise from medications and vaccines to help keep animals healthy or help them recover from injury or illness. The tumor invades the surrounding tissue, making surgery difficult. Radiation and chemotherapy serve as adjunctive therapy, yet ISS is rarely curable.  

Thomas continues searching for clues explaining why some cats develop this cancer and others don’t. Her ongoing work could provide additional guidance for cat owners and veterinarians regarding this severe complication associated with vaccination.    

Rabies: An Ancient and Ever-Present Disease   
While it’s easy for those of us in North America to take rabies for granted since it’s unlikely we, or our pets, will contract and die from the infection, not so long ago, rabies was a health threat taken very seriously in the United States. For many areas of the world, rabies remains an ever-present danger, with roughly 59,000 people (primarily children) dying each year. Estimates for how many dogs die of the disease might run into the millions, according to experts.  

Vaccination for rabies has dramatically decreased the number of cases in pets and people in the US. Rabies vaccine is one of the most effective vaccines ever developed – it must be since it must protect against a deadly virus.  

Foundation-funded researcher Dr. Darryn Knobel has been studying rabies and vaccination for a long time. As a native of South Africa, he witnessed first-hand the devastating effects of rabies in communities throughout his native country. Knobel’s work has uncovered some interesting findings.    

Knobel has conducted multiple studies in South Africa and Kenya. Rabies is a significant disease in these countries, and many communities are resource-poor when it comes to the vaccination of their community dogs. One of Dr. Knobel’s studies looked at the effect of immunization on overall mortality in puppies. He found that while rabies vaccination did There was no effect on young male puppies; however, there were higher mortality rates in female puppies for unclear reasons.    

Dr. Knobel expanded on this work in a recently completed study in Kenya that, among other research questions, wanted to see if the effects of rabies vaccination differed by sex. After vaccinating and monitoring 363 6-week-old puppies for up to seven weeks, the researchers found no detrimental effect of vaccination and found a substantial survival benefit for female puppies and either a benefit or no effect in male puppies. The team also speculated that the puppy survival effects go beyond just rabies protection from the vaccine and that other non-specific protective effects against unrelated diseases were also present (a theory that will require further investigation).

With vaccine hesitancy a major threat to both animal and human health, this research sought to replicate the previous study but in finding no detrimental effects to female puppy health with the rabies vaccine, and provide crucial, evidence-based information to combat vaccine hesitancy and enhance trust in vaccine safety.  

Partnering With Your Veterinarian to Guide Vaccine Decisions 
Veterinarians often find themselves uncomfortable trying to accurately balance the risk of vaccine reaction with the risk of disease. Owners can help by discussing their pets’ lifestyle, including contact with other animals, travel history and past vaccination history.    

By working together, owners can be active partners in deciding which vaccines are best for their pets.  

Leaders in Vaccine Research   
The Foundation has funded research to improve all animals' lives through improved vaccination strategies for decades. Donor support provides crucial funding, leading to the first canine parvovirus and feline leukemia virus vaccines, and we continue to fund studies focused on vaccine development.    

Researchers are tirelessly working on improving vaccine strategies for heartworm disease, leptospirosis, feline herpesvirus, equine strangles, and cancer vaccines, among other diseases.  

However, there’s still so much we must continue to do.  

Learn how you can support the science to ensure pet owners and veterinarians have the tools to keep pets happy, healthy and thriving.    

Resources:  

Related Stories & News