Do Cats’ Personalities Change After Vaccines? The Surprising Truth


Cat vaccines play an important role in protecting feline health, but some cat owners have concerns about potential behavior changes after vaccination. The core feline vaccines help prevent dangerous diseases like panleukopenia, calicivirus, rhinotracheitis, and rabies. While most cats tolerate vaccinations well, mild side effects can occasionally occur.

It’s not uncommon for cats to seem lethargic, have a temporary loss of appetite, or run a low fever for a day or two after vaccines. More concerning behavior changes like aggression or neurological symptoms are very rare. Most studies have found no significant association between vaccines and long-term behavior problems.

Cats may act differently after vet visits simply due to stress. Their uneasiness usually resolves on its own within a few days. Understanding the facts around vaccinations can help cat owners balance the health benefits with the very small risks.

Normal Vaccine Reactions

It’s common for cats to experience some mild side effects in the days following vaccination. These reactions are usually temporary and not cause for concern. The most common include lethargy, reduced appetite, and low-grade fever (Source).

Lethargy after vaccines is caused by the immune system ramping up to build antibodies. The vaccines introduce small amounts of antigens to stimulate an immune response. This process requires energy and can leave cats feeling more tired and inactive than usual (Source). The lethargy typically resolves within 24-48 hours.

Low-grade fever, around 1-2 degrees above normal, is also common and results from the immune activation. Like lethargy, the mild fever should pass quickly. Cats may feel slightly warm to the touch or seek out cooler areas to lay (Source).

Appetite loss may occur because cats don’t feel 100% while their immune system is busy. Making food more enticing with broths or treats can help during this time. As long as the cat is drinking water, appetite should bounce back within a day or two.

Long-Term Behavior Changes

There is little evidence that vaccines cause permanent behavior changes in cats. While some cats may experience lethargy, irritability, or other temporary changes after vaccination, these effects are generally mild and resolve within a few days (PetMD).

Some owners mistakenly attribute long-term aggression or other behavioral issues to vaccines. However, these behaviors more likely stem from genetic factors, lack of socialization, stress, underlying medical conditions, or changes in environment (Quora). While vaccines may temporarily exacerbate certain behaviors, there is no scientific evidence they permanently alter cat personality or behavior.

Vaccines work by stimulating an immune response, but do not cross the blood-brain barrier to cause neurological changes. Reactions tend to be localized and temporary. It’s important not to confuse correlation with causation when it comes to perceived behavior changes after vaccination.

Stress and Veterinary Visits

Veterinary visits can be very stressful for cats, regardless of whether vaccines are administered (Reducing the Stress of Veterinary Visits for Cats | VCA). The car ride, new environment, smells, and handling by strangers can cause anxiety and fear. This stress response can temporarily alter a cat’s behavior after the appointment.

To minimize stress, do what you can to make veterinary visits calmer experiences. Give praise and treats, use cat-friendly handling techniques, and schedule appointments when the clinic is quieter (6 Ways to Make Your Cat’s Vet Visit Less Stressful). With positive reinforcement and familiarity, most cats can become more comfortable with veterinary care over time.

Rare Adverse Reactions

Although most cats tolerate vaccines well, in very rare cases cats may experience severe adverse reactions. These include anaphylaxis and vaccine-associated sarcomas.

Anaphylaxis is a life-threatening allergic reaction that can occur within minutes after vaccination. Signs include facial swelling, hives, vomiting, diarrhea, difficulty breathing, collapse, and even death (1). Thankfully, anaphylaxis is extremely rare, occurring in an estimated 1 in 10,000 to 1 in 50,000 cats vaccinated.

Vaccine-associated sarcomas are aggressive cancers that can develop at the injection site weeks to years after vaccination. They are estimated to occur in 1-10 out of every 10,000 vaccinated cats. Risk factors include certain vaccines, injection sites, gender, and genetic predispositions. While concerning, the risk of developing a vaccine-associated sarcoma is still low compared to the dangers posed by infectious diseases like rabies, FeLV, and FIV (2).

To reduce adverse events, current vaccine guidelines recommend the minimum effective doses be given infrequently in specific locations. Still, the benefits of core vaccines far outweigh the small risk of reactions in most cats (3).




Benefits vs. Risks

Vaccines help prevent many serious and potentially fatal diseases in cats, including distemper, feline leukemia, rabies, and upper respiratory infections. According to the Cornell Feline Health Center, “Vaccination is an important part of responsible cat ownership and helps provide maximum protection against infectious diseases” (1). While no medical treatment is completely without risk, the benefits of routine vaccination in cats greatly outweigh the potential downsides.

It’s true that some cats may experience mild side effects from vaccines, such as lethargy or a low fever for a day or two. However, serious adverse reactions are very rare. On the other hand, the diseases that vaccines help prevent can lead to severe illness or death in cats. According to, distemper alone used to kill up to 90% of infected cats before vaccination became widespread (2). Today, thanks to routine vaccination protocols, these potentially fatal diseases have become uncommon in pet cats.

When weighing whether or not to vaccinate a cat, the known protective benefits clearly outweigh the small risk of side effects. By vaccinating cats, owners give them the best chance at a long and healthy life.

Alternative Vaccine Schedules

Some veterinarians are open to alternative vaccine schedules, especially for adult cats that have already received their kitten series of core vaccines. According to the American Association of Feline Practitioners vaccination guidelines, after the kitten series and 1 year booster, core vaccines may only need to be boostered every 3 years for indoor cats and low disease risk cats.1

For low-risk adult cats, some vets allow extending the rabies vaccine to every 3 years instead of annually. Others allow titers testing instead of automatic boosters. Titers measure antibodies to determine if vaccines are still providing immunity. This can prevent over-vaccination in cats.2

Discuss an individualized vaccine schedule with your vet based on your cat’s lifestyle, risk factors, and titer results. Less frequent vaccines may be an option for healthy adult cats in low-risk environments. But kittens and higher risk cats need to closely follow vaccine guidelines to ensure protection.


Lifestyle Factors

As cats age, it’s normal for their behavior and activity levels to change. According to WebMD, aging cats tend to sleep more, play less, and have reduced mobility. They may forget things like where the litter box is located or become disoriented and confused at times. Older cats can also become more vocal and attention-seeking.

Cats begin transitioning into their “senior” years around age 7-10. At this stage, they start losing interest in play and become more sedentary. Primal Pet Foods notes that cats also undergo gradual behavior changes as they reach 10-15 years old. They act less energetic, prefer routines, and gain weight more easily.

While aging itself causes natural behavior shifts, factors like diet, environment, and health issues can influence changes too. Monitoring a cat’s behavior across their lifespan can help identify abnormalities versus normal aging patterns. Any sudden, dramatic changes in an older cat’s temperament warrant a veterinary exam to check for underlying medical conditions.

Monitoring After Vaccines

It’s important to monitor your cat closely after vaccination in order to watch for any potential adverse reactions. According to the American Animal Hospital Association (AAHA) guidelines, you should monitor the vaccination site for swelling or lumps using the “3-2-1” rule (AAHA). This means checking the site 3 times on the day of vaccination, 2 times the day after, and 1 time 2 days after. Contact your vet immediately if you notice any swelling larger than 2 cm, facial swelling, or lethargy. Other signs to watch for include vomiting, diarrhea, loss of appetite, pale gums, seizures, or collapse. It’s also important to monitor your cat’s behavior and energy level for any concerning changes. While most cats will be just fine after vaccines, alerting your vet to potential problems right away allows quick treatment if needed.


While vaccines can sometimes cause mild, short-lived side effects in cats, significant or long-term behavior changes after vaccination are very rare. There is no scientific evidence showing routine vaccines lead to lasting personality changes in cats once they recover from any initial fatigue or malaise after the vaccination appointment. The risks associated with not vaccinating cats far outweigh any small risk of adverse events from vaccines themselves. As long as you keep an eye on your cat for 1-2 days after vaccination and discuss any concerns with your veterinarian, you can feel confident vaccination provides vital protection for your cat and the community.

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