The Truth About Cat Vaccines – Catmart
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The Truth About Cat Vaccines

Vaccines have gotten a bad rap in recent years. But when people talk about vaccines, they talk about vaccines for their children; so what about vaccines for cats? Are cat vaccines dangerous, and do they cause side effects? Or do they do more good than bad?

Are cat vaccines safe, and should you give them to your cat? Vaccines provide immunity to diseases that are deadly or painful for your cat to go through. At the same time, there is only a vanishingly small chance that your cat will experience a side effect. Vaccines provide protection against feline herpes virus, feline calicivirus, distemper and rabies; each of these diseases is worse than the side effects vaccines can cause. Cat vaccines do not contain mercury or formaldehyde, nor do they contain unaltered live bacteria/viruses. Ask your vet if you are hesitant to have your cat vaccinated.

The first thing we’ll look at is whether cat vaccines are safe, and how they work—with an honest look at their side effects, how bad they are, and how common they are. We’ll also detail how necessary vaccines are, and what they contain (active ingredients, adjuvants and so on). Then there’s core vaccines vs. non-core vaccines, and aftercare for cats that have had their shots.

The guide below is a long one—so get yourself a warm drink and strap in!

Are Cat Vaccines Safe?

cat vaccine
Vaccines may seem dangerous, but on closer inspection, they aren’t bad like some people think.

It’s common these days for people to be wary of vaccines. They’re an emotive subject, because most of the discussion revolves around vaccinating children, and anything that involves the safety of children is emotive. You may feel something like that with regard to vaccinating your cat.

That being said, the science behind vaccination is sound. There are risks involved, but these risks are a) well defined and b) occur only very rarely. Vaccine hesitancy stems from a misunderstanding and blurring of these risks, not to mention pseudoscience that is categorically not true.

So, without wasting any more time, let’s take a look at all the concerns people have about vaccines and how they work.

How Do Vaccines Work?

To understand what vaccines do to your cat, you first have to understand their purpose and how they work. According to the Immunisation Advisory Centre,

Edward Jenner is considered the founder of vaccinology in the West in 1796, after he inoculated a 13 year-old-boy with vaccinia virus (cowpox), and demonstrated immunity to smallpox. In 1798, the first smallpox vaccine was developed. Over the 18th and 19th centuries, systematic implementation of mass smallpox immunisation culminated in its global eradication in 1979.

All vaccines work in roughly the same way. They work in conjunction with the immune system, not against it or in its place. The process begins when the vaccine is injected. The vaccine contains a harmless version of the bacteria or virus that causes the disease being vaccinated against; so the parvo cat vaccine contains parvovirus, for example. The virus or bacteria is killed, weakened or broken down before being administered so that it won’t hurt the cat. Crucially, though, the body still recognizes the bacteria or virus as a threat. Because they’re dead or weakened, though, the immune system can easily deal with them.

On a functional level the immune system works by using antibodies. Antibodies are special cells that can detect and flag an invader like a bacterium or a viral cell. Other cells can then destroy these invaders. The body can produce antibodies for each kind of bacteria or virus that invades it, but only if it has encountered that kind of invader before. So, the point of a vaccine is to make the body recognize the parvovirus, for example, as an invader so that it’s better prepared for it next time. Then when the cat becomes infected, the body can fight it off.

Can Cat Vaccines Make My Cat Sick?

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It is possible for a vaccine to make your cat sick. But a) it won’t make your cat as sick as catching the disease from the wild, and b) the chances of this happening are very small.

It is possible for a vaccine to make your cat sick. This is a risk that is acknowledged by the scientific community; the issue is that it’s also a risk that has been misunderstood and taken to be bigger than it is.

Mild reactions to a vaccine are fairly common. These include a low level fever, lethargy and decreased appetite. These symptoms stem from the immune system’s reaction to the viral or bacterial particles that were in the vaccine: fever, for example, occurs because the body tries to kill the bacteria/virus by creating too warm an environment. These symptoms typically clear over the course of a few days, which is the time it takes for the damaged/dead ‘invaders’ to be fully processed.

You may also notice a small swelling at the site of the injection. This, again, relates to your cat’s immune system. When your cat’s body notices bacterial or viral particles somewhere, it instigates what’s called the ‘histamine response’. This is where a hormone called histamine is sent to a part of the body, which makes it swell up. The idea is that this allows more blood to the area, and consequently more antibodies (because these get from place to place through the bloodstream). More antibodies means a stronger immune response. So a slight swelling is normal too.

It’s recommended that if these symptoms continue for longer than a few days, that you take your cat to the vet.

Can Cat Vaccines Cause Allergic Reaction & Tumors?

It is possible for cat vaccines to cause an allergic reaction. According to the Cornell Feline Health Center:

In very rare cases (1-10 of every 10,000 vaccines administered), cats can have allergic reactions to vaccines. In mild cases, which constitute the majority of allergic reactions to vaccines, cats may develop hives, itchiness, redness and swelling of the eyes, lips, and neck, and mild fever. Severe allergic reactions may cause breathing difficulties, weakness, vomiting, diarrhea, pale gums, and collapse. If a cat shows any signs of allergic reaction after vaccination, contact a veterinarian immediately.

If a swelling near a vaccination site persists for more than three weeks or begins to grow, contact a veterinarian immediately. Such persistent reaction could be a sign of a type of cancer called feline injection site sarcoma (FISS). These rare tumors are believed to result from inflammation associated with vaccination, and can occur up to 10 years after vaccination in some cats. Treatment requires aggressive surgical removal of the tumor with wide borders of normal surrounding tissue. With this in mind, cats should receive vaccines in places where large amounts of tissue can be removed, such as the limbs or tail, which can be amputated in the event of FISS. Cats generally do very well after amputation of either a tail or a limb.

Again, these are known issues that are acknowledged by medical professionals and researchers. They aren’t things that have been hidden from the public, and their causes are well understood.

Risks & Side Effects vs. Benefits of Cat Vaccines

In all aspects of medicine, the doctor or vet has to make a balanced judgement. That means measuring the risk of whatever procedure they could perform against the potential benefits it might bring.

In some cases, there are no risks and lots of benefits, in which case the procedure should obviously be seen through. Giving a cat worming tablets is an example. These have only very rare side effects, and these side effects aren’t deadly; the benefit that this is weighed against is that you can deworm your cat. With such obvious benefits and unlikely risks, you won’t find a vet that says they’re worried about prescribing worming tablets.

At the opposite end of the scale is something like brain surgery. Brain surgery can easily go wrong, and healthy parts of the brain can be affected. If a cat is near its end of life and has many other health problems, the benefits of the surgery are few, as the cat is likely to pass away soon anyway; in this case, while there is a small potential benefit with lots of risks, meaning that it may be wrong to go ahead with the procedure.

Vaccines are very firmly in the former camp. They can prevent awful diseases that cause cats immense suffering, and with precious little chance of side effects. In the event of side effects occurring, they are invariably not as bad as the effects of the diseases they prevent.

Are Cat Vaccines Necessary?

Cat vaccines aren’t necessary in the sense that your cat will immediately and definitely pass away if it doesn’t have them. Both your housecat’s domestic and wild ancestors got by without them, albeit with shorter lifespans as a rule.

But they are necessary in the sense that your cat could suffer if it doesn’t have them.

Do Kittens Need to Have Vaccines?

It’s especially important that kittens get their shots as their immune systems aren’t as strong as older cats’.

Kittens are especially susceptible to disease. That’s because their immune systems aren’t yet fully developed.

Your kitten’s immune system has its roots in its mother’s milk. Incredibly, cat milk—like all mammal milk—contains antibodies that the mother passes on to its children. These antibodies then defend the kitten from whatever disease they’re intended for. They don’t act exactly like a vaccine, because they don’t teach the kitten’s immune system anything; they just last a few weeks and then are gone.

Despite that, though, kittens are still more susceptible to diseases than adult cats. That’s because they aren’t yet immune to as wide a range of diseases as adult cats. They also spend lots of time around other cats (their litter mates), meaning that diseases can easily pass from one to the other. There’s also the issue that some kittens are taken from their mothers before they’re fully done weaning, meaning that they haven’t taken a full antibody dose from their mothers through their milk.

If you’re unsure about whether it’s right to vaccinate your kitten, talk to a vet. They will tell you about the horrible diseases that the vaccines are for, and the minute chance that they could cause side effects for your kitten.

Do Adult Cats Need to Have Vaccines? (Or, How Often Should Cats Be Vaccinated?)

Cats should have most of their important vaccines early in life; it’s when a cat is a kitten that it’s most vulnerable. Not all vaccines require yearly boosters, so some vaccines will only need to be administered once.

That being said, many vaccines are more effective if booster shots are administered once every 1-3 years. The reason is that the level of antibodies for a certain illness can decrease over time if there is no exposure to it.

There are tests that can be performed to determine the level of antibodies in your cat’s blood. However, these are more expensive for you and stressful for your cat than booster shots. As such, vets recommend booster shots as a matter of course.

Besides that, if your cat never had its shots when it was a kitten, it should have them no matter what age it is. It may have already caught something like, say, feline herpes virus; but there will be diseases it hasn’t yet caught, and it’s best if it doesn’t catch them. Vaccines and booster shots will stop it from doing so.

What Do Cat Vaccines Contain?

What most people are concerned about with regard to vaccines is what they contain. That could be the fact that they contain ‘live’ versions of the bacteria or virus in question; or it could be that they have an additive/preservative which people are afraid of.

So, what do cat vaccines contain, and could those ingredients be harmful?

Are Cat Vaccines Live Viruses?

Many cat vaccines do contain live viruses. But the idea that this is a major issue is a misunderstanding of how vaccines are made and how they work.

In short, there are three different kinds of vaccine in use today. These are:

  1. Live attenuated vaccines/modified live vaccines. These vaccines do contain live viruses. But these viruses are either weakened or genetically modified so that they won’t produce disease when injected, even though they will still multiply. This is comparable to an ‘easy mode’ or ‘tutorial’ version of the disease that your cat’s body can easily fight off. Despite the virus being weaker, it will still create the same antibody response as if your cat were infected with the ‘real thing’.
  2. Killed/inactivated vaccines. These are vaccines that contain viruses or bacteria that have been killed in one of various ways. Killing the organisms in the virus means that the body still reacts to them, but not as strongly. These kinds of vaccines don’t tend to be as effective as live ones. That’s why manufacturers add things to them (adjuvants) to make them stronger.
  3. Subunit vaccines. These are vaccines that contain only parts of the infectious organism. These parts are the parts that the body needs to react to to produce an immune response, but since the organisms are broken down and dead, they don’t pose a threat to the body. They’re commonly called recombinant-DNA vaccines.

You can find cat vaccines of each of these three types. So, yes, some cat vaccines do contain live bacteria and viruses. What you won’t find are vaccines that contain whole, unaltered and deadly diseases; these were used in the early days of vaccinations, but became steadily less common and are now almost unheard of.

Do Cat Vaccines Contain Mercury?

One of the main concerns that people have with vaccines is that they contain mercury. But when you research the reality behind this statement, it’s less of an issue than people think.

First of all, vaccines don’t contain plain ‘mercury’. The substance that people are concerned about is actually called Thimerosal (also known as Thiomersal), which is an organic compound that contains mercury. There are two different kinds of mercury. According to the CDC, the two kinds…

…methylmercury and ethylmercury — are very different. Methylmercury is the type of mercury found in certain kinds of fish. At high exposure levels methylmercury can be toxic to people. In the United States, federal guidelines keep as much methylmercury as possible out of the environment and food, but over a lifetime, everyone is exposed to some methylmercury.

Thimerosal (i.e. the mercury derivative in vaccines) contains ethylmercury, which is cleared from the human body more quickly than methylmercury, and is therefore less likely to cause any harm.

Thimerosal is toxic when taken into the body, but at the level which it was found in vaccines, there was nowhere near enough to cause harm. It’s like how eating a big block of chlorine might kill you but accidentally swallowing a little swimming pool water won’t hurt you in the slightest.

Second, few vaccines contain Thimerosal anyway. It isn’t vital to the production or storage of any vaccine. It was first used in multi-dose vaccine vials in the 20th century as a preservative; a multi-dose vial contains enough doses to immunise several people/pets, and a way of keeping the vaccine mix stable and functional was needed that didn’t kill the bacteria or virus it contained. But not all vaccines are stored in multi-dose vials, with others coming in single doses. Plus there are other preservatives that can be used which don’t have negative effects either. And in July of 1999, public health agencies decided to stop using Thimerosal anyway because of peoples’ concerns; it’s now uncommon.

If you feel that you need to be absolutely clear, you should ask your vet about the ingredients that your cat’s vaccines contain. That’s because there are lots of different kinds of vaccine made by different manufacturers. But to take an example, this vaccine for FeLV contains a different preservative (Gentamicin) instead of Thimerosal.

Do Cat Vaccines Contain Formaldehyde?

Another ingredient that people have issue with is formaldehyde. That’s partly because of formaldehyde’s reputation: we’ve all heard of it, and seen it used as a kind of preservative. It also has an ugly-sounding chemical name, which doesn’t help.

But there are a couple of things to note here. The first is that formaldehyde isn’t used as an ingredient in any vaccine. Rather, it’s used in the production of a limited number of vaccines and in a limited sense, too. According to the University of Oxford’s Vaccine Knowledge Project,

It is used in the production of some vaccines to inactivate toxins from bacteria and viruses (for example, poliovirus, Hepatitis B antigen, and diphtheria and tetanus toxins). It is possible that tiny traces may remain in the 6-in-1 vaccine (Infanrix Hexa), Hepatitis B vaccine (HBVaxPro), one of the Pre-school Booster vaccines (Repevax), and the Teenage Booster vaccine (Revaxis). However, formaldehyde breaks down quickly in water (and most of the vaccine is water).

So, you could compare scaremongering about formaldehyde in vaccines to saying there’s bleach in cereal because bleach is used to clean the conveyor belt at the factory where it’s made. There is no formaldehyde in your cat’s vaccine.

Besides that, formaldehyde is actually a natural chemical. It’s produced by all living organisms including plants, animals and people—so there’s formaldehyde in both you and your cat regardless of any vaccines! Interestingly, ‘[t]he amount of natural formaldehyde in a 2-month-old infant’s blood (around 1.1 milligrams in total) is ten times greater than the amount found in any vaccine (less than 0.1 milligrams). A pear contains around 50 times more formaldehyde than is found in any vaccine.’ So, no—formaldehyde isn’t a problem for your cat.

What Vaccines Do My Cats Need?

Talk to your vet about which vaccines your cat needs.

Vaccines are classified into two different categories. These are core vaccines and non-core vaccines. Core vaccines are those which prevent diseases that cause real suffering, are highly contagious, and which no cat owner would want their cat to have. It’s considered vitally important that your cat have them, not just so that it doesn’t suffer, but so it can’t pass on those diseases to other cats.

Non-core vaccines are, as the name suggests, not quite as important as core vaccines. They are for conditions that cats can easily live with, but which will irritate your cat, cause it chronic pain or just generally make its life miserable. These vaccines should at least be considered depending on exposure risk, i.e. where your cat lives, whether it interacts with other cats, and so on.

Core Vaccines for Cats

To be clear, ‘core vaccine’ isn’t a general term. But it hasn’t come from nowhere. It was coined by the AAHA, which is the American Association of Feline Practitioners Vaccination Advisory Panel. They consider the following four vaccines to be absolutely crucial to your cat’s health.

The panluekopenia (feline distemper) vaccine. Feline distemper is another name for parvovirus infection. Parvo is a viral infection that’s spread through bodily fluids like saliva. It can be caught either from the environment, or from another cat. It’s very serious, and unless quick and correct treatment is undertaken, can result in a 50% mortality rate (or worse). What makes parvo even more of a threat to your cat is that it spreads very quickly between cats in close proximity, meaning it’s often responsible for killing entire litters of kittens. The parvo vaccine, as described above, relies on injecting small amounts of dead/weakened parvovirus that your cat’s immune system can easily fight it off. This is far safer than allowing the cat to encounter feline distemper naturally.

The feline herpesvirus (viral rhinotracheitis) vaccine. Feline herpesvirus is a serious health issue in cats. It causes upper respiratory infection, i.e. a condition that’s like a cold or a flu, except caused by this specific kind of virus. It causes all the same symptoms as a serious cold or flu, like fever, sneezing, lethargy, discharge from the eyes and nose and conjunctivitis. Like distemper, it’s highly contagious and can spread through bodily fluids, and can easily kill a litter of kittens. 

The calicivirus vaccine. Calicivirus is another kind of upper respiratory infection. Unlike other conditions, it’s termed ‘ubiquitous’. That means it’s found almost anywhere you look: if you took a random surface and pointed a very powerful telescope at it, you’d likely find calicivirus there. Because it’s ubiquitous, it means that all cats are susceptible to catching it, and the severity of its symptoms mean that this vaccine is a crucial one.

The rabies virus vaccine. Rabies is an infectious disease that your cat can catch from wild animals. Rabies affects the nervous system and is almost invariably fatal; there is no treatment. But a vaccine can stop your cat from catching it in the first place.

When you take your cat to its first checkup with the vet, they will recommend that it has at least these four vaccines. They may also recommend those in the non-core list below.

Non-Core Vaccines for Cats

According to Cornell,

The decision to vaccinate a cat with a specific non-core vaccine involves a careful assessment of the cat’s lifestyle, age, health status, exposure to other cats (and the health of these cats), vaccine history, and, in some cases medications that the cat is being treated with. With the understanding that all treatment is associated with some risk, the vaccine-specific risk must be weighed against the potential benefit that is unique to each cat’s situation. A cat may need additional vaccines depending on its risk of exposure to infectious organisms due to outdoor access, living in a shelter, or being housed in a home with infected cats.

These vaccines are not as crucial as the core ones described above. Talk to your vet about whether they are necessary for your cat.

The feline leukemia virus (FeLV) vaccine. Feline leukemia is the leading cause of viral deaths in cats. Like FPLV, FeLV spreads through bodily fluids like nasal secretions and urine. Around half of the cats that catch the condition will pass away from it and its complications in 2 1/2 years. That being said, many cats with the condition will live full and healthy lives seemingly untroubled by being infected; it’s for that reason that this isn’t considered a core vaccine.

The feline immunodeficiency virus (FIV) vaccine. Feline immunodeficiency virus is the cat equivalent of HIV/human immunodeficiency virus. It compromises your cat’s immune system and makes it susceptible to many other health conditions. Because direct contact through bite wounds is the primary method of transmission, this vaccine isn’t considered core for indoor cats. FIV vaccines also aren’t as effective as those against other conditions.

The Bordetella bronchiseptica (kennel cough) vaccine. Kennel cough is another cause of respiratory infections, so can cause coughs, sneezes, runny noses and so on. It’s passed on through direct contact with the bodily fluids of other cats (and dogs). It’s called kennel cough because it thrives in places like kennels, where lots of pets are kept together in close quarters.

The Chlamydia felis vaccine. In cats, the chlamydia bacterium causes conjunctivitis and upper respiratory tract infections. It spreads easily from one cat to another, especially when they all live together in the same home.

The feline infectious peritonitis (FIP) vaccine. FIP is a condition that is almost always fatal. It is a mutant form of the feline coronavirus. It’s rare, but it seems to spread quickly in shelters, especially under certain conditions. Vaccination for FIP isn’t as effective as vaccination against other health conditions, but your vet may still recommend it.

The dermatophytosis (ringworm) vaccine. The kind of ringworm that affects cats is the same as the one that affects people. It’s not a worm, and it’s not a kind of bacteria; ringworm is caused by a fungal infection. It’s not deadly, but it can pass from a cat to a person/from a person to a cat. That’s why it’s not considered a core vaccine.

If you talk to your vet, they will tell you which vaccinations are most important in your area.

What Vaccinations Do Indoor Cats Need—Do House Cats NEED to Be Vaccinated?

There’s less of a clear need to vaccinate your cat if it’s going to spend its entire life indoors. That’s because it won’t encounter any other cats unless you let it. There’s therefore no chance that it can catch something like, say, rabies.

But there are still good reasons to vaccinate your cat even if it will spend its whole life indoors. They include:

  • Your cat could escape, and would then be vulnerable to any and all of these diseases. Indoor cats can easily escape through open windows or doors. It would be bad enough to lose your cat, only then to find them, and they caught something like FeLV.
  • Some diseases like calicivirus don’t need to be spread through contact with other cats. They’re found on surfaces in your home regardless of whether there are other cats around.
  • You might want to get another cat. If that cat is sick, your first cat would get sick too.
  • You might change your mind and let your cat outside. Some indoor cats beg and beg and beg to be let outside. If you feel you can’t take it anymore, and let your cat out, you would expose it to any and all of these health problems.

Besides that, these vaccines are an accepted cost of getting a new pet cat. You shouldn’t expect that getting a cat only involves the cost of buying the cat itself; other costs like food, cat furniture and transport should all be factored in. The cost of an initial vet visit and the cat’s first vaccines should be too.

So… Why Do People Think Vaccines Are Bad?

This is entirely speculation, but there are a few likely reasons why people think vaccines are bad despite the evidence in their favor.

One reason is that they’re a downright medical miracle. If something as groundbreaking as a vaccine were discovered today, Nobel Prizes would rain down on whoever discovered it. Vaccines are relevant to thousands upon thousands of different diseases, many of which are deadly and with no known cure. They also almost always work and with very limited side effects. They’re so effective that they even help people who haven’t had them (through herd immunity). In a world where science has to fight hard to discover cures for cancer and ways to make heart disease better, it seems odd that something as effective and simple as a vaccine could possibly exist.

We also fear what we don’t understand. The mechanism by which vaccines work can be broken down into simple terms, but when you look at the science of how the immune system works and how vaccines can help, things get complex. Other common tropes like the use of ‘mercury’ in vaccines similarly replace complex understanding with simple misunderstanding.

Another reason is that people have always been afraid of having shots. We get our first shots when we’re young, when of course, we don’t understand why we need them. That very real fear can last until later in life, and could morph into a general dislike and distrust of all things medical.

Aftercare For Cats After Vaccinations

If your cat has just had its shots, it may have a slight fever, a slight swelling, and may be slightly lethargic. These are all common symptoms of the diseases that the vaccines vaccinate against, and it’s normal for them to occur after a shot. There are things you can do to make your cat more comfortable at this time:

  • Keep your cat in a room that’s calm and quiet. Even if your cat isn’t experiencing any side effects from the vaccine, it’s likely stressed from going to the vet. Keep things calm for the time being.
  • Keep everything your cat needs in easy reach. Ensure that your cat has quick access to its litter tray, its food bowl and its bed.
  • Keep an eye out for more severe side effects. If the swelling doesn’t go down and the fever doesn’t go away, talk to the vet. This almost never happens; if it does, a vet can put the problem right.

In all other ways, simply be sensible. Don’t pester your cat to play with it, or pet it if it doesn’t want to be pet. Allow it to relax and recover at its own pace.

Hi! My name is Jamie Fallon. I run Catmart, an online cat health and cat behavior resource. If I'm not sat in front of my PC—and I usually am—then I'm either spending time with my cats or my other half... Whoever jumps on me or asks me for food first!

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