Cancer Feline Panleukopenia Health

Vaccines & Cats: What You Need to Know

Learn all about cat vaccines, including how they work, risks, and how frequently you should vaccinate your cat.

Is your cat up to date on vaccines? Do you think you should vaccinate your cat every year? Or shouldn’t? Don’t know? You came to the right place.

To be clear, I’m talking about vaccinating cats and what I’m sharing should not be directly applied to vaccinating humans, dogs, or other living things.

As I always say, you need to make vaccination decisions based on your current situation and what you feel is best. I’m simply going to provide framework to help you make the best decisions for your cat’s health, age, lifestyle, etc. You will read this a lot in this article: Always discuss vaccinations with your vet.

Now, let’s look at the most common vaccines for cats, how they work, controversies, and look at what the research supports at this time. And of course … you’ll see lots of my cute cats throughout the article because syringes and vials are boring to look at.

The Two Most Common Core Cat Vaccines

Beaker is up to date on his vaccines. And he’s handsome … really handsome.


This combo vaccine includes:

  • Feline Viral Rhinotracheitis (FVR), the feline herpes virus aka a kitty cold.
  • Calicivirus (C) which has similar symptoms to herpes but also causes painful ulcers.
  • Panleukopenia (P), otherwise known as feline distemper or feline parvo, infects and kills rapidly dividing cells. I HATE THIS VIRUS. It typically hits the intestines, bone marrow, and lymph nodes hard, causing diarrhea, vomiting, dehydration, a high fever, anemia, and extreme lethargy. Kittens under eight weeks do not have a good survival rate, and even adults require supportive care. If you want to learn more, read this article I wrote, 10ish Things I Learned About Panleukopenia.


This virus affects the nervous system, and once a cat shows symptoms, it’s 100% fatal. Rabid cats have rabies in their saliva and can infect humans through biting. Although it’s not common, rabies still exists in the US. Most cats likely get it from killing a rabid wild animal.

How Do Vaccines Work in Cats?

After reviewing the specifics from VCA, the easiest way to explain vaccines is they are doppelgangers for microorganism, prepping the body to recognize and attack it in the future. Vaccines can cause total prevention or lessen severity and speed up recovery.

two snuggling cats
Rigsby, 7 years; Pepe 10 years

VCA outlines three types of vaccines:

  1. Modified live – Providing the longest immunity, these vaccines contain a live weakened or genetically modified organism. Modified live vaccines are not recommended for sick, immunocompromised, or pregnant cats. Most FVRCP and rabies vaccines are modified live vaccines.
  2. Killed or Inactive – These organisms are dead, dead, deasky. Since dead organisms don’t provide the same level of immunity as live ones, these vaccines have an added ingredient called an adjuvant. It’s designed to improve the immune system’s response. Make a mental note because we’ll talk about adjuvants later in the article. It’s very important, so don’t forget it. (Also, cats with feline immunodeficiency virus (FIV) or feline leukemia virus (FeLV) should get a killed FVRCP vaccine.)
  3. Subunit – This vaccine only contains a part of the organism.

Now, vaccines don’t always work, and it’s important to mention vaccine failure can occur.

Vaccine Failure

Not common, but sometimes vaccines fail for a variety of reasons, with these being the most common, according to VCA.

Different Variations

Just like the flu in humans, feline rhinotracheitis (FVR) and calicivirus (C) have many strains, and a vaccine lessens the severity and speeds recovery of these upper respiratory viruses.

Orange & white cat
Sunshine, 4 years

Panleukopenia (the P part of the FVRCP vaccine) has only one distinguishable strain, so different variations are not an issue. I’m not sure if that strain alters slightly or if your cat needs exposure with every vaccine to stay protected. I couldn’t find an answer to that in my research, but I did find a fact sheet in the Feline Journal of Medicine and Surgery that data shows immunity for 3-7 years (or possibly more) years. Since the vaccine is a combo, it’s not like the P can be removed, so this is likely a moot point right now. What this means: A vaccinated cat may still get a kitty cold, but they’ll fight it better than a non-vaccinated cat. If your cat’s up-to-date on vaccines and not a kitten, you don’t have to worry about panleukopenia (because there is one main strain).

Antibodies from Mom

When kittens are nursing, they get antibodies in their mother’s milk. While this is a good thing, those antibodies can attack a vaccine as if it’s the actual virus itself. What this means: If you have a kitten, ask your vet for the best vaccination schedule to make sure Mom’s antibodies aren’t slaying the vaccine too soon.

Stressed, Sick, or Immunocompromised

Stress and illness can impact how the vaccine works. So can age and chronic illnesses that impact the immune system. What this means: It’s best to vaccinate a healthy cat with low stress levels. If your cat has a compromised immune system (ex. from FIV) discuss the best vaccination schedule with your vet.

Side Effects

Most cats don’t have side effects to vaccines, but some do. I’ve seen kittens limp a little after receiving vaccines (because they are administered in the hind legs), but they usually are better within an hour or two.

tuxedo cat playing
Alfredo is properly vaccinated, but he has chronic upper respiratory issues. This could be from a lack of vaccinations or a slew of other reasons. He’s sweet and awesome and we love him and snot anyway.

According to Cornell University College of Veterinary Medicine, mild side effects to vaccines are similar to those of a kitty cold, and include lethargy, fever, swelling at the site, and sometimes a decreased appetite. If those symptoms do not improve within a few days or worsen, call your vet.

Very few cats have an allergic reaction, but it is possible. Cornell outlines mild allergic reactions presenting as “hives, itchiness, redness, swelling of the eyes, lips, and neck, and a mild fever.” Major allergic reactions are much worse, and similar to anaphylaxis in humans. If your cat has difficulty breathing, extreme weakness, pale gums, vomiting, diarrhea, or collapses, contact your vet immediately.

Lastly, if your cat develops a lump at the injection site that persists or grows, reach out to your vet. I’ll explain why in the next section.

Controversy, Cancer & Adjuvants

Woman holding tabby cat
Vito and me.

As I mentioned, killed or inactive vaccines include adjuvants to improve the recipient’s immune response. According to Animal Wellness, adjuvants include things like mercury, aluminum salt, sugar, gelatin. The surprising part of this is that adjuvants are possibly a problem for a small but measurable amount of cats. (Again, cats, not humans.)

The adjuvant, along with other factors like genetics, can create the cat’s immune response to be excessive, similar to an allergic response, resulting in inflammation. And what happens with inflammation? A risk for cancer. The risk is small, and Blue Pearl estimates 1 out of 10,000 vaccinated cats will develop a vaccine-induced sarcoma.

Cat with cut
Vito’s incision from his 2nd surgery.

Those odds didn’t play in my favor. In 2018, my 13-year-old cat Vito had a lump on the back of his neck. Exploratory surgery confirmed fibrosarcoma, a type of malignant cancer from an injection site. Now, the vaccine best practices are to inject a hind leg (so if there’s an adverse reaction, it’s easy to amputate). This meant Vito’s cancer was from a vaccine he received at a much younger age, because those best practices were in place.

cat in bandana
Vito received a special bandana when he graduated his 5 chemo treatments.

These tumors tend to grow slowly and don’t commonly spread by the time of diagnosis. They also never stop growing, so if they aren’t removed, the chance of spreading increases. Vito had two surgeries and five rounds of chemo before he went into remission.

What’s being done to combat this problem?

  1. There are some intranasal vaccines (yup, drops in the nose).
  2. Now, there is a non-adjuvant rabies vaccine for cats.
  3. Vaccines should be injected in the rear legs of cats, so that if there is a complication, the leg can be amputated.
  4. Greater attention paid to vaccine frequency. This means doing so based on health, age, and lifestyle.

Should You Vaccinate Kittens?

YES! Kittens absolutely need to be vaccinated to increase their immunity to common viruses. Remember when I said earlier I hate panleukopenia? It takes the lives of so many kittens, and it is totally heartbreaking. I lost one foster kitten with few symptoms, and another shortly after adoption. That cat was vaccinated, but still needed more boosters. According to VCA, kittens do not have protection until 7-10 days after their 2nd booster.

Medium haired kittens snuggling.
Sansa (formerly Jocelyn) and Drake loved cuddling.

Talk to your vet about the appropriate schedule. At the shelter where I foster, once kittens eyes open they get the FVRCP intranasal vaccine. They should receive their 1st injected FVRCP vaccine around 6 weeks. The boosters continue every 3 weeks until 16 weeks. A kitten has to be at least 3 lbs to receive the rabies vaccine. No boosters needed.

Although not considered a core vaccine, many vets recommend kittens and outdoor cats get the FeLV (feline leukemia virus) vaccine.

Adult Cats: To Vaccinate, or Not to Vaccinate?

That is the question, but if you wanted an easy answer, you’re not getting one from me – ha!

Let’s consider a few things. First, it’s important that if you don’t know a cat’s history or if you know a cat hasn’t been vaccinated, get them vaccinated as soon as possible. The benefits outweigh the risks.

Next, some studies have shown that cats vaccinated with FVRCP are still protected when older. The bottom line is, we may be over-vaccinating our cats. That doesn’t mean to stop vaccinating them, but to make smart decisions about the frequency and type of vaccines. The less times poked, the better, so ask about three-year non-adjuvant vaccines, as well as intranasal vaccines. I am on the concerned side because of what I experienced with Vito, so I’ve had honest conversations with my vet. Please do the same (if you’re concerned). Your vet should work with you to keep your cat protected but not over-vaccinated.

I Say “Yes” to Vaccinating If …

Your cat goes outdoors, whether all the time or sometimes, supervised or not. Why? Because there is always a risk of them going somewhere they shouldn’t or encountering something they shouldn’t. You may also want to consider the vaccine for FeLV (feline leukemia virus), and I would say a rabies vaccine is a must.

black and white cat in car
Lucy is around 10 years old, but I didn’t have her since she was little. I’ll likely give her vaccines for another few years unless there are clear changes to policies.

Your cat is 1) a kitten, 2) under 10 years old, or 3) they’re an adult and you don’t know their vaccination history. It’s important to make sure your cat has a strong immune system that can combat common viruses, like rhinotracheitis, or ones that are fatal without vaccination, like rabies.

You foster cats. Vaccinated cats can still carry the microorganisms that make other cats sick. The best way to offer protect in your home is to make sure all cats are properly protected. Also, be sure to follow the two-week quarantine rule for newcomers.

You live in an area with lots of bats. Bats are the top carrier of rabies in the US. If a bat gets into your house, your cat will chase it and likely catch it.

Your cat killed a bat. If your cat has been vaccinated, they’ll need to go to the vet immediately to get boostered. This offers more protection.

I Say “Maybe” to Vaccinating If …

white cat with bow tie
Dolly is good on vaccines for another two years, and then I will likely stop since she’ll be in her mid-teens by then.

Your cat is over 10-years-old, stays indoors, and has been fully vaccinated its entire life. Talk with your vet about the level of protection, but as a person who had a cat with fibrosarcoma, I definitely look at vaccines slightly differently. My 12-year-old is completely disinterested in going outside. After a lifetime of vaccinations, I’m okay forgoing vaccines for her moving forward. She received a 3-year one last year, so she’ll be in her mid-teens when it’s time to get another one, and I will likely pass.

Your cat has any sort of chronic illness, especially one that makes them immunocompromised. Again, broken record, but talk to your vet about your cat’s specific situation.

Your cat already suffers from an autoimmune disease. Talk this out with your vet, because your cat may still need vaccines. However, if your cat has a problem with inflammation, you already know their immune response is high, which means they will be at an increased risk for a injection-site sarcoma. Both cats I know that had fibrosarcoma had inflammatory bowel disease (IBD). This is not a coincidence, since similar actions in the body (overstimulated immune system) cause both.

cat with certificate
Vito is proud of his certificate after his first surgery.

I Say “No” to Vaccinating If …

Your cat has had fibrosarcoma. They are at high risk of developing it again. This isn’t even a question. No more vaccines.

So, Should You Vaccinate Your Cat?

Most likely the answer is yes, but be smart about it. I also recommend having an open and honest conversation with your vet that you want to be mindful not to over vaccinate. Ask about intranasal vaccines, 3-years vaccines, and non-adjuvant vaccines. And keep in mind, there are lots of opinions out there, but there is the fact that vaccines do prevent more fatality in cats than cause problems. It’s a personal decision you have to make and weigh the pros and cons with your kitty’s age, health, and lifestyle. Good luck!

Learn More About Cat Vaccines From People Smarter Than Me

Additives and Adjuvants in Animal Vaccines, W. Jean Dodds, DVM

Disease Information Fact Sheet: Feline Panleukopenia, Journal of Feline Medicine and Surgery.

Feline Fibrosarcoma, BluePearl Specialists

Feline Vaccines: Benefits & Risks, Cornell Feline Health Center

Vaccines: The Pros and Cons of Vaccinating Your Dog or Cat, Dr. Debora Lichtenberg, VMD

Vaccines for Cats, Ernest Ward, DVM; Rania Gollakner, BS DVM

Vaccinating Your Cat, International Cat Care

By LizsKittyBootCamp

Hi, I'm Liz, and I'm a cat behaviorist who provides advice and insights on cat behavior.

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