injection-519388_1280Revised 12/27/21

Vaccination is an ongoing controversy in veterinary medicine today. Veterinary schools and specialty organizations are promoting fewer vaccines at longer intervals, while many practitioners stubbornly cling to their annual booster schedules. Who’s right, and what’s the truth?

The Science Behind Vaccines

As a responsible pet caretaker, you probably take your animal companion to the vet every year for a check-up and “shots.” You probably get one or more reminder postcards about it! But while an annual check-up is still vitally important for your pet’s health, vaccination—how many and which ones—is a stormy controversy among veterinarians.

Until recently, vaccinations were considered harmless and beneficial. But today, scientific evidence proves that there are many potential harmful effects.

Most common pet vaccines are made using a “modified live virus” (MLV), which means that the virus is alive and can replicate in the animal’s body, but has been modified so it does not cause disease–or, at least, not severe disease. This natural replication produces excellent, long-lasting immunity. However, some diseases are too dangerous to allow to replicate, or for some other reason the organism (whether virus, bacteria, or other parasite) must be killed to use in a vaccine. In that case, because the immune response is not as good, other agents, called “adjuvants,” are added to the vaccine to stimulate a more vigorous immune response. The newest generation of vaccines includes recombinant vaccines that use only small fragments of the disease agent, which are inserted into a harmless carrier virus.

The purpose of a vaccine is to stimulate the body to produce antibodies (germ-fighting proteins made by specialized white blood cells) and “memory” cells, so that if the animal ever encounters the disease again, his system will be primed and ready to defend against it. It turns out that many vaccines are so good at this that they don’t need to be boostered every year, if at all.

Most veterinary schools and several veterinary associations have studied the research, and concluded that annual boosters are unnecessary for most pets, and potentially harmful in some. They now recommend waiting at least three years between boosters for the most common dog and cat vaccines.

“Herd Immunity”

The real purpose of vaccinations is not only to protect your particular pet against disease, but to establish immunity in enough animals so that if a disease does break out, there will not be enough susceptible animals to sustain an epidemic. This is called “herd immunity” since the idea was first developed for livestock.

Because more and more people are not getting their animals (and often their children) vaccinated at all, herd immunity is weakening in many areas. Recent outbreaks of distemper in dogs and cats, and whooping cough and measles in children, are of great concern. No vaccines can be just as dangerous as too many vaccines; the basic kitten and puppy shots (distemper, parvo and rabies) are still needed—see “Recommendations” at the end of this article.

Annual Boosters: An Unnecessary Danger

Immunologically, annual boosters are totally unnecessary for the vast majority of animals. The way the immune system works, a vaccine does not suddenly lose its effectiveness a year and a day after the last shot, although some veterinarians imply that being even a tiny bit late for a booster is putting your animal at grave risk. It’s just not true. If a vaccine produces adequate antibodies, and those antibodies are still in the blood the following year, they will actually inactivate the booster, making it worthless as a protector, and potentially a danger. Additional vaccine doses in an immunized animal do not increase or improve the immunity, but they do increase the risks.

Dr. Ronald Schultz at the University of Wisconsin is the premier vaccine researcher in the country. He says, that “canine distemper and adenovirus-2 vaccines both provide good lifelong immunity. These need not be given annually.” He says the same about the feline distemper (panleukopenia) vaccine. He believes that a single dose of modified live vaccine given at 10-14 weeks of age is protective for life. Both canine and feline distemper vaccines have been shown to induce immunity for 3-8 years or more. Moreover, all of these are diseases of young animals; most older animals are naturally resistant. Once vaccinated, adult animals are typically fully immune.

For cats, the danger is real. There have been many studies on the FVRCP vaccine (feline viral rhinotraceitis, calicivirus and panleukopenia). This vaccine is cultured and grown on feline kidney cells. Over-vaccination might increase the risk of inducing the production of antibodies that bind to kidney tissues. (1)

The rabies vaccine is required by law for most animals in most jurisdictions. Therefore, it is important from a legal standpoint to follow your jurisdiction’s regulations concerning rabies vaccines for your pets. Killed rabies vaccines are labeled for either 1 or 3 years; but the vaccine in the bottle is exactly the same in both cases. Giving a 3-year vaccine every year puts your pet at even greater risk for an adverse reaction. If your pet receives a killed rabies vaccine, be certain that it is labeled for 3 years. For cats, there is a safer vaccine: Purevax, by Merial, is a recombinant vector vaccine that does not induce the inflammation that occurs with killed, adjuvanted vaccines. If your vet clinic does not carry it, ask if they will order it, or try to find one who does. It’s worth it.

Rabies is a serious disease, and 100% fatal (with the exception of one young woman that has never been duplicated). Cats are frequently infected. Even indoor cats need to be protected from rabies; bats can get into attics, come down chimneys, or enter through windows; in one case, a cat caught a bat on the balcony of his 14th floor apartment in downtown Denver. There are serious legal liabilities as well, with life-threatening potential for your cat apart from the disease. Rabies vaccines provide long-lasting protection if given properly to young animals.

If your pet has a chronic disease or is for any reason not completely healthy, it should NOT be vaccinated. All vaccines carry a label that states “for use in healthy animals only.” Many jurisdictions will accept an exemption letter from your veterinarian attesting to the fact that your pet is at high risk and cannot be vaccinated.

Risks Associated with Vaccination

Several known risks are associated with one or more specific vaccines.

Autoantibodies. Antibodies are the goal of vaccination. We want the body to produce antibodies against the disease being vaccinated against. However, the vaccine manufacturing process contains some quirks that cause the body to make antibodies to a wide variety of components in the vaccine. Most vaccines are produced through a culture medium such as eggs, blood serum, or certain types of cells. The organisms are grown in these nutritious cultures, then filtered for manufacture into vaccines.

While the filters are small enough to keep out whole cells, both viruses and a variety of loose proteins will end up in the final product. When injected, the dog or cat’s body then makes antibodies to many of the proteins as well as the virus itself. Studies at Purdue University showed that canine vaccines grown in calf serum caused antibodies to be made to many calf proteins including red blood cells; thyroid; DNA; connective tissue proteins such as collagen, fibronectin, laminin, albumin, transferrin; and others.

Unfortunately, calf proteins are so similar to dog proteins that the antibodies react to the dog’s own tissue as well—this is an “autoimmune” reaction (“auto” means “self”). Every vaccinated puppy developed multiple autoantibodies, and every additional booster produced even more autoantibodies. Because the puppies in the Purdue study were euthanized at 22 weeks of age, it is unknown if these autoantibodies would lead to disease, but follow-up studies with client-owned dogs are ongoing.

Diseases associated with the tissues to which autoantibodies are made include thyroiditis (which can leads to hypothyroidism in dogs), lupus, cardiomyopathy, and autoimmune hemolytic anemia. In cats, chronic renal failure appears to be linked to over-vaccination with the FVRCP vaccine.

Researchers say that “systemic autoimmunity appears to be the inevitable consequence of over-stimulating the host’s immune ‘system’ by repeated immunization with antigen.” (2) (Emphasis added.)

Feline Chronic Renal Failure (CRF). The common feline distemper (panleukopenia) virus is grown in a culture of feline kidney cells. Recent work at Colorado State University showed that most kittens developed autoantibodies to their own kidney tissues after being vaccinated for distemper. When autoantibodies react with body tissue, the result is inflammation. Each booster vaccine creates even more antibodies—and more inflammation. Chronic low-grade inflammation is the primary cause of CRF, which is almost guaranteed to develop in older cats. The authors of the study suggest a causal relationship between the distemper vaccine and the development of CRF. In other words, annual re-vaccination for feline distemper may be the major cause of death in older cats. Annual boosters for feline distemper are totally unnecessary because the immunity produced by the initial kitten vaccines is so long lasting.

Feline Injection-Site Sarcomas (FISS),
formerly known as Vaccine-Associated Sarcomas (VAS). (The name was changed so vaccine manufacturers wouldn’t be blamed; although vaccines certainly are the primary cause.) Many people have heard about the malignant, fatal tumors called fibrosarcomas that can be caused by some vaccines in cats. This cancer occurs in the connective tissue. Research in dogs shows that vaccines cause autoantibodies to be made to many connective tissue components.

The two vaccines currently implicated in cats are rabies and feline leukemia. . What do these products have in common? They are killed vaccines spiked with “adjuvants”–compounds that increase the immune system’s response to the vaccine. Unfortunately, in cats, this additional response includes inflammation that can lead to the formation of cancer. Any injection of any kind (antibiotics, steroids, fluids, etc.) can trigger tumor formation. However, additional vaccines are the most likely to significantly increase the risk of cancer. Merial has non-adjuvant vaccines for feline leukemia and rabies. If your veterinarian doesn’t have them, find a vet who does. Cats should never receive killed vaccines of any kind. 

The incidence of FISS is at least 1 in 10,000 cats, and some studies suggest it may be as high as 1 in 1,000 cats. Let’s think about that number for a minute. There are 90 million cats in U.S. homes today; so between 90,000 and 900,000 (almost a million!) of them will develop a fatal cancer caused by vaccination. There may be a genetic susceptibility to this cancer, but there is no way to check for this genetic defect. What if one of those cats is yours?

When vaccines were given between the shoulder blades, these cancers were inoperable because they would grow into the spine, ribcage, and chest. This became such a serious problem that it is recommended to give the rabies vaccine in the right hind leg, and leukemia in the left hind leg. This is so that, when a tumor does develop, the whole leg can be amputated and the cat’s life can be saved. (3) These recommendations were published in 2013, yet I still see cats being vaccinated between the shoulders.

Recently, the same vaccine-associated tumors have been reported in both dogs and ferrets. While these species are more resistant, obviously they are not immune.

Titer Testing to Assess Immunity

A “titer” measures antibodies in the blood for a specific disease. The blood is diluted, then incubated with a marker of the disease (proteins from a virus, for instance); if antibodies are present, they will react with the marker and produce a positive titer. The most extreme dilution at which the reaction is seen is notated. For example, there may be a reaction to the distemper virus at a dilution factor of 1:1200, which would be a strong positive.

Titers have gained some acceptance as a guideline on whether an animal needs a booster vaccine. For instance, if your cat had a high titer to distemper, there would be no need to give a distemper booster. There is a growing body of scientific evidence that most viral vaccines produce high, long-lasting titers in most animals.

However, while titer tests are available for many common diseases, there is no consensus on how high a titer is protective for each disease. For example, a positive rabies titer of 1:2 is very low and would not be proof of immunity to rabies. (In most jurisdictions, titers are not legal proof of rabies immunity, and even a high titer will not exempt the animal from vaccination for rabies as required by law.) While low or borderline titers are open to interpretation, most experts agree that a high titer indicates good protection from the disease.

Additionally, a titer measures only one facet of immunity: circulating antibodies. When a vaccine is given, antibodies are produced by the lymphocytes, but in addition to antibodies, particular lymphocytes called “memory” cells are produced. These memory cells live quietly in the lymph nodes until there is another exposure to their particular disease. These memory cells then “jump-start” antibody production, so that the reaction is much faster and stronger than it would be if the body had never been exposed to the disease before.

Also not measured by a titer is the animal’s “cellular immunity.” This involves T-lymphocytes that directly attack and kill viruses, bacteria, and other invading organisms. This is another “first line” immune response; it starts immediately and keeps going while the body produces enough antibodies to take over and clear the infection.

If you wish to minimize your pet’s vaccines, but want to know—to the extent possible—whether or not she is protected against disease, it may be worthwhile to have your veterinarian run a titer test. It is not perfect, but it does provide a snapshot of at least one important part of the immune system.

The main benefit of a titer is to separate responders (animals with a positive titer) from non-responders. Non-responders have an immune defect that prevents them from generating any antibodies at all to the vaccine. Re-vaccinating these animals does not change their status, and only increases the risk of adverse effects without any benefit at all.

NOTE: You only need to titer once. Checking titers every year, or every three years, is unnecessary. Essentially, if your pet has one positive titer, at any level, at any time, then she is a responder, and you’re done.


Experts agree that only “core” vaccines—panleukopenia and rabies for cats, and distemper/parvo and rabies for dogs—are truly necessary. All other vaccines are optional, and should only be given if the animal’s lifestyle or health considerations make them necessary.

Indoor cats do not need vaccines against Feline Infectious Peritonitis (FIP), Giardia, Ringworm, Bordatellaor Feline Immunodeficiency Virus (FIV, also called “feline AIDS”). Even if cats do go outside (something I don’t recommend!), the risk of contracting these diseases is very small.

Dog vaccines available include several that are worthless or dangerous. Coronavirus, Giardia, and Ringworm vaccines either do not prevent disease, or the disease is so mild as to not be worth vaccinating for. Rattlesnake, Bordatella (kennel cough), and Leptospirosis should only be given to high-risk animals or when required, such as for show dogs or boarding kennels. Leptospirosis is becoming more common throughout the U.S.—although the vaccine is not 100% effective, and causes some of the most severe reactions. Dogs in limited areas of the northeastern U.S. may need the Lyme disease vaccine.

An initial two- or three- vaccine series for kittens and puppies is necessary for most vaccines to be effective. Vaccines should be given at least 3 weeks apart; there is little published research suggesting an outside limit, but at least a 4-8 week interval is probably safe and effective. The animal should be at least 8 (preferably 9) weeks of age before vaccinating. The common breeders’ practice of vaccinating pups at 2 weeks of age and then every 2 weeks is useless, not to mention harmful to young puppies’ immune systems.

The initial canine distemper/parvo/hepatitis/parainfluenza or feline panleukopenia–rhinotracheitis (feline herpesvirus)–calicivirus vaccine can be given at 8-9 weeks of age, followed by a booster at 12-14 weeks and, if desired, 1 year later. However, according to immunology researcher Dr. Ronald Schultz at the University of Wisconsin, older kittens and puppies, and adults of unknown vaccination status, will be protected by a single vaccine, regardless of prior history. Many protocols now suggest re-vaccination for distemper/parvo or panleukopenia every 3 years, but even this is overly cautious. Given the general resistance of adult animals to viral infection, plus the potential for the feline panleukopenia vaccine to contribute to the development of chronic renal failure, minimal re-vaccination, if any, seems wise.

Cat vaccines pose a particular problem due to the feline tendency to develop cancer at the site of any trauma or injection–even subcutaneous fluids.

Feline Leukemia is not very contagious, and adult cats are naturally resistant. Experts suggest that kittens should receive the initial series of two vaccines to protect them during the most susceptible period, and a one-year booster. It need never be given again.

Chlamydia is an uncommon cause of respiratory infections in cats, which are usually due to feline herpesvirus and/or calicivirus. The Chlamydia vaccine does not prevent infection; and at best reduces (but does not eliminate) symptoms and viral shedding. Like most bacterial vaccines, it requires two shots to be of any use; it does not produce long-lasting immunity; and it carries a higher risk of adverse reactions. It is neither necessary nor recommended.

Rabies vaccination is required by law in many states and municipalities, and must be up-to-date to transport animals from state to state. Check with your local animal control to find out what is needed in your area. For cats the Merial Purevax recombinant vaccines can be used in place of the older killed vaccines that cause injection-site tumors.


Vaccination is a medical procedure with risks and benefits. Ask your veterinarian what the purpose of each recommended vaccine is; why your animal companion should receive it—based on current health status, lifestyle, and risk factors; and what the potential adverse effects are. Make sure that every vaccine is well justified, and don’t let anyone bully you into complying with their clinic’s outdated or inflexible protocols. Don’t give multiple vaccines at the same time; space them out at least 3 weeks apart. You have the information; now you must become an active partner in your pet’s health!


(a) Songaksorn N, Petsophonsakul W, Pringroa K, et al. Prevalence of autoantibodies that bind to kidney tissues in cats and association risk with antibodies to feline viral rhinotracheitis, calicivirus, and panleukopenia. Journal of Veterinary Science. 2021 May;22(3):e38.

(2) Tsumiyama K, Miyazaki Y, Shiozawa S.  Self-organized criticality theory of autoimmunity. PloS one. 2009; 4(12), e8382.

(3) Scherk MA, Ford RB, Gaskell RM, et al. 2013 AAFP Feline Vaccination Advisory Panel Report. Journal of Feline Medicine and Surgery. 2013 Sep;15(9):785-808. doi: 10.1177/1098612X13500429.