Vaccines for Cats: We Need to Stop Overvaccinating
Lisa A. Pierson, DVM


A vaccination is a preparation of microorganisms (pathogens), such as viruses or bacteria, that is administered to produce or increase immunity to a particular disease. There can be no disputing that vaccines save lives but they also have the potential to cause serious side effects which will be discussed on this webpage.

Before we get started on this discussion, it is important to understand that there is no single vaccine protocol that fits every situation and every person’s individual comfort level. There are many factors involved in the decision making process but at the core of each decision is:

a risk/benefit analysis and

consideration of the duration of immunity (DOI) information that is available to us.

This webpage discusses vaccine protocols that are within my comfort zone but may not be within the readers’. It is also important to understand that a discussion involving FVRCP (herpes, calici, panleukopenia) and FeLV (feline leukemia) vaccination is separate from one discussing rabies vaccination. This is because rabies vaccination is legally required in many areas of the world.

Regarding DOI data: Unfortunately, many veterinarians are not considering the facts and are recommending unnecessary vaccinations.

The subject of vaccine administration is one of the most controversial topics in human and veterinary medical literature, making it a common area of debate – and stressful decision-making – among parents and pet owners.

Given that this is an area of controversy, I want to start with a ‘food for thought’ question:

How often are you getting vaccinated for measles, mumps, chicken pox, tetanus, etc.? Yearly? Every 3 years?

I doubt it.

So why aren’t more people questioning the reminder cards that many veterinarians send out asking for the pet to be brought in for yearly vaccines?

More to the heart of the matter, why are many veterinarians ignoring the current vaccine guidelines which call for a longer period of time between vaccine administration than has been the ‘standard’ for many years?

The evidence-based recommendation/suggestion to vaccinate less frequently than we have been doing for the past many years came out of Colorado State University approximately 18 years ago so this is not something new.

These ‘newer’ guidelines are based on DOI (duration of immunity) studies showing that it is not necessary to vaccinate cats as frequently as they have been in the past. In fact, the DOI studies show that it is not even necessary to vaccinate as frequently as every 3 years for FVRCP.

It is very important to understand that the current guidelines state that the FVRCP combination vaccine should not be given more frequently than every 3 years. That is not the same as stating “these vaccines should be administered every 3 years.”

This is a significant point of confusion among some veterinarians and most lay people. The guidelines are worded in such a way as to invite vaccine intervals that are even longer than 3 years.

A cat’s immune system is not any more ‘forgetful’ than a human’s immune system. In other words, there is no reason to believe that they need to be vaccinated so often. Their immune system, to the contrary, has a very good memory.

For many years, humans have thought of vaccines as ‘all good and no bad’ but that line of thought cannot be further from the truth. Given the serious side effects that can manifest themselves after a vaccine has been given, people need to start applying more critical thought when making decisions about vaccination protocols.

It is very important to understand that no vaccine is 100% safe.

However, it is also very important to understand that vaccines save lives and there can be no debating that fact.

These two facts, along with other factors discussed on this page, enter into every decision we make regarding how we vaccinate our cats.

I wish that I felt comfortable saying “ask your veterinarian for the best advice regarding the vaccination of your cats” but I don’t.

As noted above, many of my colleagues are simply not taking the time to carefully peruse the scientific literature that provides DOI data showing that we are over-vaccinating many of our pets. (Rabies will be discussed separately.)

This webpage is not intended to be a comprehensive discussion on all matters involving vaccinations but, instead, will cover some vaccine basics, and my personal views on the subject – including how I vaccinate my own cats.

Please note that even some (all?) of the experts who sat on the panel that came up with the AAFP (American Association of Feline Practitioners) vaccine suggestions, as well as one veterinarian who is head of the vaccine division of a major vaccine-producing company, do not vaccinate their own animals as frequently as their guidelines/suggestions/package label states. (source: personal communication)

These veterinarians acknowledge that the current suggestions/package labels do not reflect the fact that challenge studies have shown a very long duration of immunity (DOI) – lifelong, for some diseases – from just a single, properly-timed, vaccine.

Unfortunately, it has been hard enough to get veterinarians to switch from annual vaccines to the current 3-year protocol so it is going to be an uphill battle to get them to vaccinate even less frequently. Therefore, I do not see changes in the AAFP suggestions coming anytime soon.

To put this in perspective, note that the recommendation to go to a 3-year vaccine protocol came out of Colorado State University more than 18 years ago, yet there are still many (~50%) veterinarians administering annual vaccines.

This reluctance to change is especially true of the older generation of veterinarians (myself included having been involved in this profession for over 40 years) who lived through a time when the mortality rate from rabies, distemper, etc., was very high. Vaccines came along and saved lives – no question – but it is time to start paying more attention to the current DOI studies – some of which have been available for many years.

I urge the reader to take the time to do their own research into this area and not necessarily rely only on your veterinarian’s recommendations. It will be up to the reader to decide how they want to handle vaccine administration in their own kittens and adult cats for FVRCP and FeLV.

My goals in writing this page are to get the reader to:

stop blindly over-vaccinating their cats

apply more critical thought – including reading the studies


Yes, I am shouting about the last issue.

Adjuvants are substances that are added to vaccines to purposely cause inflammation at the vaccine site in order to alert the immune system to its presence. They are used with killed vaccines to stimulate a more robust immune response but can also cause a fatal, aggressive tumor (sarcoma) at the site of vaccine injection. (See below for a picture of “Chicken” – a sweet cat who lost her battle with this cancer.)

To be fair, adjuvanted vaccines are not the only substances that can cause sarcomas. Even non-adjuvanted (modified life) vaccines, as well as other injectable drugs, can cause these tumors. This is why the acronym “VAS” (Vaccine Associated Sarcoma) is being dropped in favor of “ISS” (Injection Site Sarcoma).

That said, at this time, it appears that adjuvanted vaccines have a higher risk rate of sarcomas when compared with non-adjuvanted vaccines.

Do not assume that your vet is using non-adjuvanted vaccines. ASK before allowing any vaccine to be administered to your cat.

To repeat much of what I have said above: There is nothing in the scientific literature to support annual vaccination with the FVRCP and Feline Leukemia (FeLV) vaccines. It is well-known that:

the vaccines commonly used for cats confer immunity for much longer than 1 year – and actually provide lifelong immunity in most instances for panleukopenia;

adjuvants contained in killed vaccines put cats at risk for fatal tumors (sarcomas);

even the non-adjuvanted FVRCP vaccines have caused sarcomas, as have the PureVax (non-adjuvanted) vaccines;

natural immunity to feline leukemia is very strong by the time the cat reaches ~1 year of age; and

there may be a link between the FVRCP vaccine and kidney inflammation.

Please note that kidney disease is the most common subject that I consult on and it is considered by many to be the number one cause – or at least a very common cause – of death in our older cats.

The diseases we most commonly vaccinate cats for are caused by viruses – not bacteria. While it is difficult to induce long-term immunity to bacterial infections, vaccines targeted toward viruses are usually more efficient at conferring long-term immunity in the recipient. Please keep this in mind as you read about vaccine frequency below.

Keep in mind that a vaccine protocol is not a ‘one size fits all’ issue and that the medical community is lacking in definitive research in many areas of vaccinology. This is why the AAFP and myself make suggestions regarding vaccine protocols – versus etched-in-stone statements.

Everyone has a different ‘take’ on a risk-benefit analysis and people have to work within their own comfort zone. What follows are suggestions that work within my comfort zone.

There are 5 viral diseases that cats are commonly vaccinated for:

herpes (rhinotracheitis) – the ‘R’ in FVRCP

calici – the ‘C’ in FVRCP

panleukopenia (“feline distemper”) – the ‘P’ in FVRCP

feline leukemia – FeLV


Please do not vaccinate for FIV (Feline Immunodeficiency Virus – aka “feline AIDS”), FIP (Feline Infectious Peritonitis), bordatella, giardia, or chlamydia.

Keep in mind that if you do decide to vaccinate for FIV (an adjuvanted, and very ineffective, vaccine), your cat will now test ‘positive’ since the FIV test cannot tell the difference between an infected cat and a vaccinated cat.

Chlamydia vaccines used to be routinely administered but this organism is no longer considered to be a ‘core’ pathogen. Therefore, vaccination for chlamydia should only be considered in situations where the need can be substantiated through testing.


Most people are familiar with the abbreviation ‘FVRCP’ which stands for Feline Viral Rhinotracheitis (herpes), Calici, Panleukopenia. FVRCP is a combination vaccine that includes 3 out of the 5 vaccines that will be discussed on this webpage.

This vaccine can either be modified live (all ML vaccines are non-adjuvanted) or killed (adjuvanted). The route of delivery can be either injectable or intranasal.

In general, only use a modified live (NON-adjuvanted) – never a killed (adjuvanted) – FVRCP vaccine, with injectable (not intranasal) being the preferred route of administration in most, but not all, instances.

Herpes and calici: These are the upper respiratory viruses that can cause watery/swollen/ulcerated eyes (mainly herpes), sneezing, nasal congestion, and oral ulcerations. The severity of illness ranges from very mild to severe but the mortality rate is very low.

Unfortunately, as is true for the ‘common cold’ in humans, there is no 100% effective vaccine for herpes and calici in the cat. One reason is that these viruses mutate (change) frequently and there are many different strains. The vaccine will not prevent infection but will, hopefully, lessen the severity of clinical signs.

Panleukopenia: This is a highly contagious virus that infects the intestines causing severe bloody diarrhea and vomiting. This disease has a very high mortality rate.

Do not let your kitten or cat go unprotected from panleukopenia. This virus can cause a very cruel death.

My suggestions:


Vaccinate kittens with FVRCP twice starting at 8-9 weeks of age with the second, and final kitten vaccine, administered when the kitten is no younger than 16 weeks of age.

The AAFP guidelines state that you can start this vaccine when the kitten is as young as 6 weeks of age but, unless there is a very high index of risk, I would definitely not vaccinate a kitten this young.

We wait until the kitten is at least 16 weeks old to receive his last kitten shot because the antibodies he got from nursing on his mother will have decreased to a low enough level that his own body can respond to the vaccine in order to make his own antibodies. (Maternal antibodies within the kitten can ‘tie up’ the vaccine before his body has a chance to respond to it.)

The AAFP guidelines suggest giving the FVRCP every 3-4 weeks until the kitten is 16 weeks of age. This is done in an attempt to vaccinate the kitten the minute his maternal antibody level wanes to a low enough level to allow him to respond to the vaccine. That way, there will be a minimal gap between the time his mother’s antibodies stop protecting him and the time when he can start making his own antibodies.

That said, I would rather not give this many vaccines to a kitten. Unless there is a high index of risk, I prefer to limit it to 2 vaccines total for the kitten series.

If you read the AAFP guidelines, there is the potential for administering 4-5 vaccines in the kitten series. Given the fact that the FVRCP vaccine has been proven to cause kidney inflammation (nephritis), I am not comfortable following their suggestions.

See Membranoproliferative glomerulonephritis associated with over-vaccination in a cocker spaniel puppy at the bottom of this webpage.

The above case involved an owner who, without veterinary supervision, vaccinated his puppy 7 times – 1 time per month. The puppy died at 7 months of age from kidney failure due to kidney inflammation. The two most striking facts/comments in this case report are:

“In addition, antigens in the complexes were similar to the vaccine antigens in the DHLPP vaccine, suggesting that the glomerulonephropathy in this puppy was secondary to frequent and unnecessary vaccination.”


“Further studies are required at this time to determine the role, if any, that recent past and current vaccine protocols play in the development of protein-losing nephropathies.”

The last statement is very important considering the fact that chronic kidney disease is the most common subject that I consult on and that 2/3 of the kidney cells must be non-functioning before we see any elevation in blood markers such as BUN and creatinine. Therefore, we certainly may be damaging kitten kidneys by giving them 4 vaccines by the time they are 4 months old but not be aware of it.

Put another way – if 7 vaccines in 7 months resulted in the death of a puppy, then I am not comfortable with 4 vaccines within 10 weeks for a kitten.

Young adults:

The AAFP guidelines suggest giving a FVRCP booster 1 year after the last kitten vaccine – i.e. – when the cat is ~ 16 months of age. However, if the kitten responds as he should to the kitten series, this booster should not be needed. The rationale behind the 1 year booster shot is to cover any kitten that did not properly respond to the kitten series.

Reasons why a kitten may not fully respond to a series of vaccines as a kitten and would benefit from a 1 year booster are:

1) The last kitten shot was given when he was younger than 16 weeks of age.

2) Maternal antibodies hung on longer than 16 weeks and interfered with his immune system’s ability to respond to the last vaccine. (We have good studies showing that the maternal antibody levels are low enough in most kittens to allow them to respond to a vaccine by the time they are 8-12 weeks of age so this is an improbable scenario.)

3) The kitten was in poor health when vaccinated and did not respond properly. (Vaccines should never be administered to sick animals but, unfortunately, unhealthy animals are vaccinated more often than you may think.)

4) The vaccine was of inadequate immunogenicity which means that the vaccine was damaged in terms of its efficacy. This could happen due to a problem within the manufacturing process or because of poor handling of the vaccine after it left the manufacturing plant.

Note that some cats are genetically ‘non-responders’ and never will respond to a vaccine no matter how many you give them. In these rare cases, giving a booster vaccine 1 year after the last kitten vaccine would be of no benefit.

Deciding to give a booster vaccine 1 year after the last kitten vaccine, or not, is a judgment call.

The older a kitten is (past 16 weeks of age) when he receives his last kitten shot, the less inclined I would be to give a booster shot 1 year later.

This is because the older he is, the more mature his immune system is – and better able to respond – and the less chance there will be for the maternal antibodies to be at a high enough level to interfere with his ability to respond to a vaccine.

Another option would be to test his titer (antibody level) to panleukopenia (not herpes or calici) to help you make a decision. (More on titer testing below.) This is what I would personally opt for.


I do not repeat the FVRCP vaccine past the kitten shots – or past the 1-year booster as discussed above.

We certainly must stop vaccinating with FVRCP every year but taking it one step further, I do not follow the AAFP guidelines which suggest giving the FVRCP every 3 years since the risks outweigh the benefits.


Consider the facts that enter into the risk-benefit analysis:

1) A single, properly-timed, FVRCP vaccine confers life-long immunity to panleukopenia (the most serious disease among the 3 that the FVRCP targets) in the vast majority of cats. Those very few cats that may not be protected are considered to be ‘non-responders’ and giving them more vaccines is unlikely to help.

2) Herpes and calici vaccines lack the ability to induce complete protection. At best, they will only reduce the severity of some symptoms but will not prevent infection with these viruses and will not protect the recipient from all symptoms of disease.

3) Herpes and calici viral infections do not have a high mortality rate. Death from these viruses is extremely rare and, if it did occur, it would most likely happen in kittenhood.

4) Even though a non-adjuvanted FVRCP vaccine is much less likely to cause a sarcoma, sarcomas have been noted with these vaccines.

5) The FVRCP vaccine has been shown to cause kidney inflammation. Given how common chronic kidney disease is in the cat, this fact influences my vaccine decisions.

If you rescue/adopt an altered (spayed/neutered) adult with an unknown vaccine history, I strongly suggest running a titer for panleukopenia rather than vaccinating blindly. If the cat is spayed or neutered, chances are she or he was vaccinated as a kitten. However, the age at which he received his last kitten vaccine (past 16 weeks of age?) will not be known so the decision to administer a vaccine, or not, is a judgment call. This is a situation where titer testing can help out.

As odd as it may sound, if I rescue a neutered adult male cat with a fairly good size (empty) scrotal sac, I assume (rightly or wrongly….) that he was most likely not neutered before the age of 16 weeks. (Male cats that are neutered very young have a very flat scrotal sac.) Given that most cats are vaccinated when they are neutered, this gives us a hint (and nothing more than that) that the cat may very well have received a vaccine past the age of 16 weeks.

Also, keep in mind that many of these stray cats have been ‘traveling the streets’ and may have a good dose of natural immunity to some feline diseases although that cannot be counted on definitively.

The AAFP guidelines call for a series of 2 FVRCP vaccines to be given 3-4 weeks apart to an adult with an unknown vaccination history but the WSAVA (World Small Animal Veterinary Association) recommends that only 1 FVRCP vaccine be given with a booster vaccine 1 year later.

In lieu of this 1 year booster, I would suggest titer testing.

Studies have shown that cats over 16 weeks of age with a healthy immune system respond very well to just 1 FVRCP vaccine.

Feline Leukemia (FeLV):

Feline leukemia (the disease that results from the feline leukemia virus versus a primary cancer), is a complicated disease. It typically attacks the bone marrow of the cat but cats vary in their response to the virus. Some cats clear the virus from their system and become FeLV ‘negative,’ some cats live for many years with the virus in their body but are not symptomatic, and some cats become ill and die within a few years of becoming infected.

FeLV is NOT highly contagious and its transmission requires prolonged intimate (i.e., sharing food/water, mutual grooming) contact with an infected cat. Natural immunity is very strong in most cats by the age of 1 year. AAFP guidelines suggest vaccinating all kittens but the vaccine is recommended in adults only if they will be in contact with a known FeLV positive cat – which would be a very rare situation.

I disagree with the recommendation to vaccinate all kittens. None of my own cats have ever been vaccinated for FeLV – not even as kittens – since they reside indoors and will not be in contact with a FeLV positive cat.

My suggestions:

Do not vaccinate kittens for FeLV unless your kitten is going to be outside (rarely a safe place to be for any kitten or cat) or is going to be housed with an FeLV positive kitten or cat.

Do not vaccinate adult cats for FeLV – even if they have access to the outdoors – since natural immunity to this disease is very strong by the time the cat is ~1 year of age. If an adult cat is going to be living with a FeLV positive cat, then vaccination should be considered.

If you are more comfortable vaccinating a cat that goes outside, please do not vaccinate him yearly. Vaccinating one time with a PureVax (the only NON-adjuvanted option) vaccine would fit within my comfort zone.


This is a very serious disease with nearly 100% mortality in animals and humans. (Only a handful of people have survived a rabies infection.) Since humans can contract rabies from animals, including cats, vaccinating cats for rabies is required by law in some areas of the country.

Any cat that is exposed to the outdoors, or any indoor cat that could possibly come in contact with a bat, should be vaccinated for rabies.

Bat rabies is the most common strain to infect humans. Bats can live in attics, fly down chimneys, come in through windows, and ‘indoor-only’ cats have been exposed to rabid bats on balconies, as well as within their own home.

To reiterate what has been said elsewhere on this webpage, I would never inject an adjuvant into any cat in my care because of the increased risk of an injection site sarcoma (very aggressive, and usually fatal, cancer).

Fortunately, there is one line of non-adjuvanted rabies vaccines available and that is Merial’s PureVax rabies vaccines but it is important to keep in mind that sarcomas have occurred at the site of PureVax vaccines so we want to be mindful of the frequency that even this vaccine is used.

Up until July 2014 we only had one choice of PureVax rabies – one that was only labeled for 1-year. It did not make me happy to have to inject feline patients with a rabies vaccine every year but the good news is that we now have two choices:

1) 1-year label

2) 3-year label

The choice is very clear for me – I will be using/recommending the PureVax 3-year product going forward.

Vaccine Types

There are 4 main categories of vaccines:


I will not use.

All killed vaccines are adjuvanted and may cause malignant tumors.

All FeLV vaccines, except for the PureVax FeLV vaccine, are killed.

Some FVRCP vaccines are killed.

Killed vaccines do not stimulate the immune system as efficiently as modified live vaccines.

Modified live (MLV)

MLV stands for Modified Live Virus. They are all NON-adjuvanted. The viruses contained in these vaccines are not killed but are attenuated (blunted) so that they will replicate in the recipient but, hopefully, will not cause disease. Most, but not all, FVRCP vaccines are modified live.

‘Reverting to virulence’ means that the virus contained in the vaccine is now infective enough to cause disease. This is a rare risk of using MLV vaccines.

Note: In 1999, I was involved in a situation where several kittens in a group died from panleukopenia post vaccination with a MLV FVRCP vaccine. The vaccine was from a leading vaccine manufacturing company and after speaking with their head veterinarian, it was determined that the vaccine may have reverted to virulence.

This means that the vaccine could have caused the very disease we were trying to protect these kittens from. Since MLV vaccines do carry this (extremely rare) risk, as well as having the potential to incite kidney inflammation, I try to limit the kitten series to 2 MLV vaccines.

Intranasal (IN)

Intranasal vaccines are also modified live viruses and all are NON-adjuvanted. They are administered via the nose and eyes. I do not use them for several reasons. The following comments pertain to the intranasal FVRCP vaccine – keeping in mind that the most important virus among the 3 that any FVRCP vaccine targets is panleukopenia:

1) The route of infection for panleukopenia is oral, not via the respiratory tract. Intranasal vaccines are better at conferring immunity for respiratory viruses and are less effective than an injectable MLV for stimulating immunity to panleukopenia.

I would never count on an IN vaccine to fully protect a patient from panleukopenia.

2) There are no DOI challenge studies for panleukopenia post-intranasal vaccination like there are for the MLV vaccines.

3) Even though they may be more efficacious for the herpes and calici viruses, they often cause sneezing and watery eyes and the owner needs to be aware of this. Our rescue group tried using these vaccines and it was a disaster because our kittens had to sit out from adoptions for about 2 weeks due to sneezing and watery eyes.

4) On a positive note, intranasal vaccines cannot cause a Vaccine Associated Sarcoma and they have been shown to cause NO kidney inflammation. However, even in light of these ‘pluses’ I am not comfortable using them to protect kittens against panleukopenia.

5) In some situations (catteries and shelters with a severe herpes/calici problem), the IN vaccine that contains only the herpes and calici viruses but not panleukopenia virus is of value since it stimulates a local mucosal (the lining of the respiratory tract) immunity very quickly and is not affected by maternal antibodies. Therefore, it can be given to kittens younger than the traditional 8 weeks of age.


Merial’s PureVax rabies (1 ml dose) and feline leukemia (0.25 ml dose) vaccines are recombinant vaccines which means they contain only a portion of the genetic material of a pathogen (virus). Therefore, reversion to virulence (able to cause disease) is impossible.

The PureVax FeLV vaccine uses a very low volume (0.25 ml) and it is injected intradermally (within the skin) versus under the skin.

Although I have not seen any literature on the negative reaction rate, I am going to assume that the recombinant vaccines will be less apt to cause an anaphylactic reaction because they contain fewer potential allergens.

They also do not replicate in the recipient and there is no reason to believe that they cause kidney inflammation.

To clarify: Merial makes a PureVax FVRCP vaccine but it is not a recombinant product. It is a modified live vaccine (non-adjuvanted) just like other manufactures make.

Recombinant vaccines appear to be the safest type available. While they can cause sarcomas, they are less likely to do so when compared to a killed/adjuvanted vaccine.