Side Effects of Vaccine on Cats (By Vaccine): Our Vet Explains

Side Effects of Vaccine on Cats (By Vaccine): Our Vet Explains

Nobody enjoys getting a shot, but vaccines have a crucial role in maintaining our health and well-being, as well as that of our feline companions. By protecting cats against common infectious diseases, vaccines have saved countless lives and helped prevent the spread of illness. However, as with any medical intervention, vaccines can come with potential side effects. Let’s explore possible side effects in cats, recommended vaccines, and ways to help reduce the risk of adverse effects. By being a well-informed pet parent, you can confidently make decisions to ensure the best possible care for your furry family member. 

Why we should vaccinate our cats

Vaccinating your cat is an essential part of being a responsible pet owner. Vaccines are necessary for several reasons: 

  • Protect against infectious disease
  • Prevent disease transmission 
  • Reduce severity of symptoms
  • Decrease long-term costs of veterinary care
  • It may be required by law (for example rabies vaccines)

What are the common side effects of vaccines on cats?

Vaccines work by stimulating the immune system to protect against specific infectious diseases. They typically contain a weakened, inactivated, or partial form of a virus, bacterium, or other disease-causing agent. When a vaccine is introduced into the body, the immune system recognizes these foreign components (called antigens) as potential threats, mounts an immune response, and produces antibodies against them. The immune system essentially becomes trained and will be able to respond quickly and effectively if the individual is later exposed to the actual disease-causing agent. 

Vaccine side effects can result from various factors related to the immune response caused by vaccination, components of the vaccine itself, and individual characteristics of the cat being vaccinated. The most common side effects of vaccination are mild and generally resolve within 1-2 days after vaccination. If symptoms last for more than 2 days, alert your vet. These mild reactions include: 

  • Soreness or swelling at the injection site
  • Mild fever
  • Lethargy or decreased appetite
  • Sneezing or coughing after an intranasal vaccine

In rare cases, more serious vaccine reactions can arise. Anaphylaxis is the most severe reaction that can occur immediately after vaccination and requires immediate veterinary attention. Cats who have had severe reactions to vaccines in the past will require a discussion with their vet about which vaccines are necessary, premedication with antihistamines and glucocorticoids, and close monitoring after vaccination. Contact your vet immediately if any of these signs develop after vaccination: 

  • Vomiting or diarrhea
  • Hives and itchy skin
  • Facial swelling
  • Difficulty breathing
  • Collapse

Lastly, some cats can develop long-term side effects from vaccination, the most significant of which is injection site sarcoma. These are malignant tumors that can arise at the site of injection. While the cause is not fully understood, it is thought that inflammation from the vaccine may play a role. Injection site sarcomas can develop between two months and 10 years after vaccination. They are typically treated aggressively with surgery, chemo, and radiation. To help with prevention and early detection, veterinarians typically follow injection site protocols (for example, always administering the rabies vaccine in the right rear limb), follow the recommended vaccination schedules to avoid over-vaccinating, and may use feline-specific vaccines. Vaccines should also be administered as far down on a limb as possible in case amputation is necessary. Your vet may recommend further evaluation of a lump at your cat’s vaccines site if it follows the 3,2,1 rule

  • Still present 3 months after vaccination 
  • Larger than 2 cm in diameter
  • Increasing in size 1 month after vaccination 

What percentage of cats experience side effects?

The percentage of cats experiencing vaccine side effects can vary depending on the specific vaccine and individual cat. In general, most cats do not experience significant side effects from vaccines, and the benefits of vaccination far outweigh the risks. According to the American Animal Hospital Association and the American Veterinary Medical Association, adverse reactions within 30 days of vaccination were reported in 0.52% of cats vaccinated. The most commonly reported side effects were lethargy, decreased appetite, inflammation at the injection site, and mild fever. Anaphylaxis occurred in approximately 1-5 per 10,000 vaccinations, and injection site sarcoma in 1 out of 10,000 to 30,000.  

Side effects for most popular cat vaccines

Cats may receive a combination of core and non-core vaccines. Core vaccines are recommended for all cats, while non-core vaccines are recommended based on an individual cat’s lifestyle and risk of exposure (for example, if they are an indoor vs. outdoor cat). It is important to follow an appropriate vaccine schedule to make sure that your cat is fully protected. 

While any vaccine has the potential to cause side effects, vaccines containing adjuvants, such as some FeLV and rabies vaccines, have been associated with higher incidence of injection site sarcomas. Adjuvants are substances added to some vaccines to enhance the immune response. There are now non-adjuvanted vaccines available for cats to help minimize these rare but serious side effects. 

Feline core vaccines


The rabies vaccine is vital for protecting cats from the deadly rabies virus, which affects the central nervous system and can be transmitted to humans. In many areas, the rabies vaccination is legally required. 

Feline Herpesvirus 1, Feline Calicivirus and Feline Panleukopenia Virus Vaccines (FVRCP)

This group of diseases includes common upper respiratory and gastrointestinal viruses that are highly contagious and can cause significant disease and death. 


Now considered a core vaccine for kittens, it protects against feline leukemia virus, which can cause immune suppression, anemia, and cancer in cats. This is especially critical for cats who go outdoors. 

Feline non-core vaccines 


This vaccine protects against a bacterial infection that can cause respiratory symptoms. This vaccine may be recommended for young cats in multi-cat environments. 


This vaccine guards against a bacterial infection that can cause conjunctivitis and upper respiratory symptoms in cats. It is not consistently administered unless in high-risk situations, as it has been associated with a higher (3%) risk of side effects. 

Other vaccines

Other vaccines are generally not recommended by the American Association of Feline Practitioners. 

How severe these side effects are on cats

Several factors can influence the severity of vaccine side effects in cats, including: 

  • Individual immune response and genetic predisposition
  • Age – Younger and older cats may have more sensitive immune systems.
  • History of vaccine reaction 
  • Overall health – Cats with compromised immune systems or other pre-existing health conditions may be more susceptible to side effects.
  • Vaccine type – for example, adjuvanted vs non-adjuvanted vaccines
  • Vaccination schedule – including frequency and number of vaccines given at one time
  • Administration technique and cooperation of the cat getting vaccinated 

So what can you do to help prevent side effects in your cat? Discuss your cat’s vaccination plan and any concerns you may have with your vet. They will be able to put your mind at ease, let you know which vaccines are most important for your cat, and when they should be administered. 

How long do these side effects typically last?

Mild side effects are usually self-limiting and should resolve within 1-2 days after vaccination. 

Can a side effect observed after a cat vaccine ever be a reason to visit the vet?

If signs of a mild vaccine reaction do not resolve within a couple days after vaccination, it may be time to contact your vet. Signs of anaphylaxis such as vomiting, diarrhea, facial swelling, hives, difficulty breathing, or collapse, constitute an emergency and require immediate veterinary attention. Additionally, long-term side effects, such as a lump at the vaccine site that is not resolving, will also require veterinary care. 

What can the vet do to help? About the vet visit

If you bring your cat to the vet with concerns about a vaccine reaction, they will start by gathering a history and performing a nose-to-tail physical exam. If your cat was vaccinated elsewhere, be prepared to answer questions about what vaccine theyreceived, when it was given, and where on their body the vaccine was injected, as well as the clinical signs that you are noticing. 

The treatment (and associated cost) for vaccine reactions in cats will depend on the type and severity of side effects, but may include: 

  • Monitoring, pain medication, subcutaneous fluids, and a bland diet for mild symptoms 
  • Antihistamines and glucocortidoicds for moderate reactions
  • Lifesaving measures, including epinephrine for anaphylaxis 
  • Biopsy, surgery, chemotherapy, and radiation for injection site sarcoma

It is important to contact your vet if you suspect your cat is experiencing a vaccine reaction. They will be able to guide you on the best course of action, monitor your cat’s recovery, and discuss any adjustments to the vaccine schedule or plan in the future. 

Frequently Asked Questions

How long can these side effects last on my cat?

Most mild vaccine side effects should resolve in 1-2 days after vaccination. If you notice side effects lasting beyond that time frame, please contact your vet. Signs of anaphylaxis, such as vomiting, diarrhea, facial swelling, hives, difficulty breathing, or collapse, constitute an emergency. 

Can these side effects lead to the death of my cat if not managed on time?

In rare cases, vaccine side effects can lead to death. This may be due to anaphylaxis if a cat is not able to receive immediate veterinary attention, or as a result of long-term side effects such as injection site sarcoma. While these side effects can be extremely scary for pet parents, it is important to remember that they are rare and that the benefits of vaccines far outweigh the risks. 


  • Dr Liza Cahn, Veterinarian

    Dr. Cahn graduated in 2013 with a Doctorate of Veterinary Medicine (DVM) from from Michigan State University. She loves working with dogs and cats and educating owners on all aspects of veterinary medicine, especially animal behavior and dermatology. Dr. Cahn has an active veterinary license in Washington State.

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