Feline Vaccination. The Rare Side Effect You Should Know About


Vaccines are preparations that stimulate an immune response against specific diseases to protect against them in the future (Vaccine Definition & Meaning). They typically contain weakened or inactive forms of disease-causing microorganisms that trigger the body to produce antibodies as if it has been infected. Vaccination is an important way to protect cats against harmful and potentially fatal infectious diseases like rabies, feline leukemia, and feline panleukopenia (Vaccines and immunization).

While vaccines are generally safe and effective, in rare cases cats may experience side effects. Understanding the potential risks can help cat owners make informed decisions about vaccination schedules. This article will examine a rare but serious vaccine side effect in cats called vaccine-associated sarcoma.

Common Vaccines for Cats

There are a set of core vaccines that all cats should receive (AAHA):

  • Rabies vaccine – Protects against the rabies virus which can cause fatal neurological disease.
  • Panleukopenia vaccine – Protects against panleukopenia virus which causes feline distemper, a life-threatening disease.
  • Rhinotracheitis vaccine – Protects against feline herpesvirus infection causing upper respiratory infection.
  • Calicivirus vaccine – Protects against calicivirus infection causing upper respiratory disease and oral ulcers.

There are also non-core vaccines that may be recommended in certain situations like the feline leukemia virus (FeLV) vaccine for outdoor cats (All Pets Vet).

Potential Minor Side Effects

The most common side effects of cat vaccines are minor and temporary. This includes lethargy, reduced appetite, and mild fever, which may occur within the first day or two after vaccination. PetMD notes that these are normal immune responses as the body produces antibodies against the vaccine antigens. Fatigue, soreness or swelling at the injection site are also potential side effects that resolve quickly. These mild vaccine reactions indicate that the cat’s immune system is responding properly. While some cats may experience no side effects, owners can monitor for these minor symptoms and provide soothing care such as rest and gentle massages until they pass within a day or two.

Rare but Serious Side Effects

While the majority of vaccine reactions in cats are minor, there are some very rare but serious side effects that can occur. These include anaphylaxis, facial swelling, respiratory distress, shock, and even death in extremely rare cases.

According to one Canadian study, the total vaccine-associated adverse event rate was 51.6 per 10,000 cats vaccinated. Most of these reactions were mild, but 0.32% of cats had life-threatening vaccine reactions. A different review in the US and Canada found rates of 0.3 cases of serious vaccine reactions per 10,000 vaccines given.

While concerning, these statistics show that severe life-threatening reactions are still extremely rare, occurring in less than 1 in 10,000 vaccinations. The benefits of protection against deadly infectious diseases often outweigh such small risks. However, it’s important for cat owners to monitor for any concerning symptoms post-vaccination and notify their veterinarian immediately of severe reactions.

Vaccine-Associated Sarcoma

Vaccine-associated sarcoma (VAS) is a rare but serious potential side effect that can develop in cats at sites where vaccines are administered. VAS is a type of aggressive fibrosarcoma tumor that arises 3-12 months after vaccination.

Incidence rates for VAS are estimated to be around 1 in 1000 to 1 in 10,000 vaccinated cats, according to studies. This means for every 10,000 vaccinated cats, around 1-10 may develop VAS. Although the risk is low, VAS accounts for up to 20% of skin and subcutaneous tumors seen in cats [1].

Minimizing Risk

There are steps cat owners can take to minimize the risks associated with vaccinations. One key consideration is splitting up vaccines and administering them separately instead of giving multiple vaccines at once. Giving one vaccine at a time reduces inflammation and potential adverse reactions (source). Vaccines should be spaced 2-4 weeks apart. Another strategy is to separate the distemper and upper respiratory combo vaccine from the rabies vaccine by 2-4 weeks.

Proper injection technique and site selection also play a role. Subcutaneous (under the skin) administration carries less risk than intravenous or intramuscular injections. The area between the shoulder blades is ideal as it limits mobility to reduce irritation. The rear limb is sometimes used but not recommended due to increased discomfort. Owners should avoid administering vaccines themselves and leave it to their veterinarian. Proper restraint, needle size, and injection angle all factor into minimizing reactions (source).

Managing Reactions

Vaccine reactions can occur within hours or days after vaccination. Mild reactions are common and may include fatigue, decreased appetite, low-grade fever, and swelling or tenderness at the injection site. These mild reactions typically resolve on their own within a day or two. However, some cats may experience more severe reactions.

Signs of a severe vaccine reaction can include facial swelling, hives, vomiting, diarrhea, difficulty breathing, pale gums, collapse, or anaphylactic shock. If any of these signs occur, take your cat to the veterinarian immediately. Severe reactions are medical emergencies and can be fatal if not treated promptly.

To manage vaccine reactions, your veterinarian may administer medications like antihistamines, steroids, or epinephrine. Supportive care such as intravenous fluids may also be given. In the case of facial swelling, cold compresses can help reduce inflammation. Make sure to monitor your cat closely in the days following vaccination.

Some steps you can take to try to prevent reactions include:

  • Only vaccinating healthy cats, not those who are sick or immunocompromised
  • Administering vaccines separately instead of in combination
  • Using injectable instead of intranasal vaccines when possible
  • Pre-treating with antihistamines for cats with prior reactions

While reactions are rare, prompt veterinary care is essential for the best outcome. Knowing the signs and being prepared can help manage vaccine reactions in cats.




Balancing Risks vs Benefits

While rare side effects like vaccine-associated sarcomas are concerning, experts overwhelmingly agree that the benefits of vaccinating cats far outweigh the small risks involved. According to Cornell University’s College of Veterinary Medicine, “Vaccination protects all cats by making disease transmission less likely; No vaccine is 100 percent effective, and the effectiveness of different vaccines varies.” They note that serious diseases like distemper and rabies can be fatal for cats, so preventing them through vaccination is crucial for population health [1].

When working with your veterinarian, be sure to discuss any concerns about reactions so you can make an informed decision together. Your vet may recommend alternate vaccination schedules or additional precautions to minimize risks for your individual cat. With open communication and preventative care, the benefits can far outweigh the potential downsides. Ultimately, regularly vaccinating your cat provides the best protection against harmful, contagious diseases that could endanger their health and be fatal.

Alternate Vaccine Schedules

Some veterinarians recommend alternate vaccine schedules for cats that are at higher risk of adverse reactions. This involves spacing out vaccines more than the typical yearly or three-year intervals, and limiting the number of antigen combinations given at once Vaccines for Cats – VCA Animal Hospitals. One approach is to separate the core vaccines like rabies, panleukopenia virus, rhinotracheitis, and calicivirus and give them 3-4 weeks apart. Vaccines for non-core diseases like leukemia can also be delayed until the cat is older. High-risk cats may only receive rabies and panleukopenia vaccines every 3 years. Discussing the benefits vs risks with your veterinarian can help determine if an alternate schedule makes sense for your cat.


Although vaccination can very rarely lead to serious side effects in cats, the benefits still far outweigh the risks. Vaccines help prevent many dangerous and potentially fatal diseases like rabies, panleukopenia, and feline leukemia. By vaccinating cats according to established protocols, pet owners can protect the health of their feline companions and avoid outbreaks of preventable illnesses. Any side effects that do occur are usually mild and transitory. In the exceptionally rare cases of severe vaccine reactions like sarcomas, early detection and treatment can greatly improve outcomes. While an individualized approach to vaccination may be warranted for some cats, completely foregoing vaccines poses even greater risks from deadly infectious diseases. Overall, routine vaccination remains the safest and most responsible choice to keep cats healthy and safe from preventable illness.

Scroll to Top