Cat vaccines can feel like a big topic to wrap your head around. But it’s important for us fur-moms and fur-dads to be up to speed when it comes to our kitties’ health, ensuring we are doing everything we can to be proactive in protecting their wellbeing. Especially when it comes to dealing with cat vaccine side effects.

Cat vaccinations are one of the most important medical treatments we will all encounter during our fur-friends’ lifetime.

Yet when it comes to vaccines, the majority of us rely on our veterinarians’ guidance on which ones our cats should receive and how often.

With all the medical lingo and high emotional stakes, it can be easy to feel overwhelmed. Especially when faced with questions concerning feline leukemia or FVRCP vaccine side effects (or other cat cancers). 

Which is precisely why we’ve written this article…to help empower the cat community to better understand cat vaccines and how to deal with any side effects, including feline injection site sarcomas (FISS) and if your cat is sick after vaccines.

Ultimate Guide to Cat Vaccine Side Effects

Here’s a summary of what you can expect from this article.

Key takeaways on cat vaccine side effects:

  • Vaccines work by stimulating cat’s immune system to recognize and fight particular microorganism
  • The majority of vaccines are administered by injection
  • Only 1-10 out of every 10,000 cats experience a serious side effect from vaccines
  • FISS are malignant skin tumors, with many studies indicating that FISS appear most frequently at vaccination sites
  • If you’re concerned about a lump on your kitty, then refer to the 3-2-1 rule

⚠️ Brittany, Paul, and the Fluffy Kitty team are not veterinarians. This article is based on facts, research, and personal experiences. We provide you with the best information we can but it is the reader’s prerogative as to what consequential action or inaction they may take. Paul and Brittany (and Yoda) shall in no event be held liable for any loss or other damages including but not limited to special, incidental, consequential, or any other damages.

Now that’s all cleared up, let’s dive in!

The Most Common Types of Cat Vaccines

Vaccines are medically and scientifically proven to help prevent insidious diseases in our fluffy kitties, supporting their overall health and life longevity. Which is why it’s so important that we know what vaccines they’re having, and why. 

The most common feline vaccines outlined by the American Association of Feline Practitioners (AAFP) are:

  • Feline leukemia (or FeLV)
  • FVRCP
    • Feline Viral Rhinotracheitis (FVR) 
    • Calicivirus (C) – together with FVR known as ‘Cat Flu’, and
    • Panleukopenia (P) – also known as ‘Feline Distemper’
  • Rabies
  • Bordetella

Kittens typically first get their vaccinations when they are 6 to 8 weeks old, with a second booster set when they’re 3 months old. From here, booster shots are generally administered a year later. 

Adult cats usually have booster shots every year or every 3 years, depending on the specific vaccine used or based on the assessed lifestyle risk. 

How Cat Vaccines Work

Without getting too caught up in the medical lingo, cat vaccinations work by stimulating a cat’s immune system to recognize and fight particular microorganisms — such as an infectious organism, virus or bacteria. 

The desired outcome from a vaccine is essentially protection from infectious diseases, as the body has been primed to protect against future infections.

The most common types of vaccines are:

  • Modified live vaccines: contain live organisms (weakened or genetically modified) that won’t produce the disease but will multiply in the cat’s body. These vaccines typically induce a longer-lasting immunity
  • Inactivated vaccines: also known as ‘killed’ vaccines, here the organisms have been killed through various processes. Killed vaccines may also have an adjuvant (added ingredient) to help boost the immunity 
  • Subunit vaccines: also known as ‘recombinant-DNA’ vaccines, here only certain parts of an infectious organism are included in the vaccine

The majority of vaccines are administered by injection, with the most common injection sites being the shoulder, lower back and hind limb, however some vaccines can be intranasal (administered via drops in the nose). 

What Are the Common Cat Vaccination Side Effects?  

It is completely understandable that you might be worried about your cat vomiting after a vaccine, for example, or the side effects of the rabies vaccine in cats. 

However, research does indicate that most cats will not demonstrate any side effects from a vaccine.

In fact, approximately 1-10 out of every 10,000 vaccinated cats will experience a serious side effect…which when put in context, is a very small percentage.

(But it’s equally important to note that mild symptoms won’t always be reported by cat owners, which makes it difficult to categorically state that all cats won’t experience side effects.) 

Possible cat vaccine side effects

  • Lethargy
  • Slight fever
  • Lumps/bumps or swelling/redness around injection site
  • Loss of appetite
  • Sneezing (especially if intranasal)
  • Vomiting
  • Facial swelling
  • Itching
  • Diarrhea
  • Hives

In terms of the window in which these side effects can typically manifest, it can either happen quickly or up to 48 hours after the vaccination. These symptoms should generally last one or two days.

Cat vaccination side effects like lethargy are more frequent. Whereas cat diarrhea after vaccination, vomiting, facial swelling, itching and hives are more severe side effects, and these symptoms would be best to closely monitor. 

So if you’re wondering whether ‘my cat is vomiting after vaccination’ is something to be worried about…our advice is that if this symptom or any symptoms worsen, or breathing difficulties arise, contact a local vet for advice.  

It’s also important to note that unusual reactions can also occur up to 3 days later.

Unusual cat vaccine side effects

  • Respiratory difficulty
  • Polyarthritis (i.e. lameness)
  • Allergic anaphylactic reaction

And in extreme cases…

Rare cat vaccine side effects

  • Cancer formation (including FISS, the cancer that led to Yoda’s amputation)
  • Collapse 
  • Death 

Most resources do suggest that these instances are in the minority and highly uncommon.

As we mentioned before, if you’re at all concerned about a cat reaction to the vaccine, then call a trusted veterinarian who’ll be able to provide more personalized advice on how to treat any side effects.

Signs to look out for

  • Cat still isn’t back ‘to their usual self’ after a couple of days
  • Lumps growing bigger or showing signs of inflammation, oozing or infection
  • Lumps that are painful and/or haven’t gone away after a week
  • Any symptoms becoming more severe

In extreme circumstances, an emergency veterinarian should be contacted. 

If your kitty does ever experience any symptoms after a vaccine, even if they’re incredibly mild, it would be useful to make a note in your cat’s very own medical passport. That way, you can mention it to your vet on your next visit for future awareness.

Feline Injection Site Sarcoma — What Is It?   

As we mentioned above, one of the possible side effects of cat injections is feline injection site sarcomas (FISS) with the most common injection site sarcoma being fibrosarcoma

FISS are malignant skin tumors of mesenchymal origin, with many studies indicating that FISS appear most frequently at vaccination sites. This is because where injections induce tissue inflammation, this leads to neoplastic transformation. 

FISS is also suggested to be more common when Adjuvanted vaccines are used, and especially where FeLV and rabies vaccines are administered.

(Research does suggest that other factors may be involved in FISS — such as pre-existing genetic and medical conditions. So it is again difficult to categorically state that FISS is solely caused by specific injections or vaccines, although there is an evident correlation.) 

The time between an injection/vaccination and the tumor developing is stated to be between 3 months to 4 years, but can be as late as 10 years post-vaccination.

Treatment for FISS generally involves surgery, radiotherapy and chemotherapy. Left untreated, cats are sadly likely to die due to complications associated with the tumor.

If you or someone you know is going through a similar experience, please send them our guide to coping with feline cancer.

If you’re concerned about a lump on your kitty, then referring to the 3-2-1 rule may be useful – as this typically guides when you should continue to monitor the lump at home versus when a biopsy may be warranted.

The 3-2-1 Rule

  • If the lump persists for 3 months or more after injection
  • If the lump is more than 2cm in diameter
  • If the lump continues to grow in size 1 month after injection/vaccine is given

When our own Fluffy Yoda was diagnosed with FISS, and we were considering the different treatment avenues, several factors we personally had to consider were:

  • Yoda’s quality of life if left untreated vs his quality of life if he had surgery
  • The associated costs of Yoda’s treatment
  • The physical requirements of traveling to Yoda’s numerous medical appointments

This was a majorly emotional time for us. But ultimately, we made the decision to commit to surgery where Yoda’s hind leg would be removed, followed by intensive post-operative treatment. 

We knew that Yoda’s sarcoma would reduce his overall lifespan, but we felt that treatment was the best option to ensure our boy was as happy and healthy for as long as possible.

If you would like to read more about our experience with treating Yoda’s FISS, and to learn more about feline cancer, then you may want to check out these articles:

Should I Still Vaccinate My Cat?

Remember, any medical procedure does carry the risk of negative side effects — for humans and felines alike! Yet when it comes to the question of whether you should still vaccinate your cat…the resounding answer from experts is ‘yes’.  

Of course, the decision is ultimately yours, but we should always take steps to offer our kitties the best quality of care and life possible. 

Likewise, asking your veterinarian the following questions about a specific vaccine can offer peace of mind, whilst helping to prevent FISS too:

  • Is the vaccination necessary?
    Consider the area you live in, your cat’s lifestyle, their age and breed. You may want to check out these vaccination guidelines
  • Is the vaccine live?
    Avoiding vaccines that use Adjuvants may be more beneficial in preventing FISS
  • Is the vaccine being administered in a standardized area?
    This will help your veterinarian to track whether a type of vaccine impacts a specific area of your cat’s body, whilst also preventing inflammation from multiple injections to a single site. Vaccines to lower limbs and the tail may also be more beneficial, so that if a tumor does develop and amputation is required — this won’t impact the central body  

Overall, most professionals maintain that the benefits of vaccination greatly outweigh the potential risks

For example, the rabies and FVRCP distemper vaccines are considered necessary core vaccines, whether your cat is indoor or outdoor.

You should also read up on local laws in your area, as some cat vaccinations are required by law in certain states.

Final Thoughts on Cat Vaccine Side Effects 

There you have it — an overview of potential cat vaccine side effects, including FISS and what to do if you’re ever concerned about your kitty’s health following a vaccination.

Has your cat ever experienced any of these side effects? We’d love to hear your story, so make sure to reach out.


P.S. If you’d love to catch-up on how Yoda is doing as a tripod cat, then check out our Instagram here.

The Fluffy Kitty

The Fluffy Kitty blog inspires cat owners around the world to live a more adventurous and eco-friendly life with their cats. Read more about our story on our about us page.

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