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Pet Vaccinations: Understanding Vaccinations for Your Cat or Dog

WebMD discusses pet vaccinations, including why pets need them, vaccination reactions, and more.

By Bill Hendrick
WebMD Pet Health Feature

Reviewed by Katherine Snyder, DVM, Dip ACVIM

Many pet owners and some animal scientists believe that we are over vaccinating our pets. They also think that some shots may be doing more harm than good. One type of cancer in cats, for example, is known to be caused by injections. In addition, vaccines can cause allergic reactions.

Because reports and rumors of side effects have become so widespread, pet owners increasingly are asking their vets about shots. Andy Smith, DVM, a long-time Atlanta veterinarian, says he has “this conversation with a client twice a week. It’s clear there’s a lot of confusion and concern.” So WebMD went to some top veterinary experts to find answers you can use in sorting out your own concerns.

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Why do pets need vaccines?

Vaccines protect against contagious, potentially fatal diseases, says Margret Casal, DMV, PhD. Casal is associate professor of medical genetics at the School of Veterinary Medicine, University of Pennsylvania. Vaccines trigger immune responses, she says, and prepare pets to fight future infections.

Casal tells WebMD that vaccines have saved millions of pet lives. And even though some once common diseases are now rare, she says veterinary groups are in agreement about the vaccines that are still necessary.

Is there a vaccination controversy?

Yes, says Andrea Looney, DVM, of Cornell University. Some experts advocate yearly shots, others every three years, and a few believe no more vaccines are needed after the first year. She says it’s similar to controversies over human vaccines. “There’s a lot of talk,” she says, “but no evidence [of widespread harm].”

Casal says fears sparked by this “over vaccination” controversy have led many pet owners to skip shots for preventable diseases, causing an alarming rise in pet deaths.

So should all dogs and cats still be vaccinated?

“Absolutely,” says Ronald Schultz, DVM, a pioneer in clinical immunology at the University of Wisconsin-Madison. But Schulz is also very much in agreement with those who say pets are being over vaccinated, calling it a “serious problem.” Often, he says, pets are vaccinated by vets who just want to keep clients coming in. But too many vaccines, especially when given in “combo shots,” can “assault” immune systems. He says some vaccines are cures in search of a disease.

Is it true that vaccines can even cause cancer?

In cats, definitely, says Richard Ford, DVM, professor of veterinary medicine at North Carolina State University. Ford says most but not all scientists believe the culprit is a chemical called an “adjuvant” that’s added to some feline vaccines. “Many [scientists] strongly recommend to avoid using any cat vaccine that is labeled ‘killed’ or ‘inactivated.’ All feline vaccines labeled in this way contain adjuvant. Vaccine labeled ‘attenuated’ or ‘recombinant’ does not contain adjuvant.”

Years ago, vets started noticing tumors forming in the area between the shoulders, where cats are vaccinated. The tumors are rare, occurring in 1 in 1,000 to 1 in 10,000 cats. Veterinarians now give this type of vaccine low on a cats' front or hind legs so they can amputate if a tumor develops, potentially saving the cats' life.

Vaccines can definitely cause cancer, says Luci T. Dimick, DVM, of The Ohio State University. She says feline leukemia is caused by a virus, and it’s listed as a “non-core” disease. Yet many vets feel kittens should be immunized even though it’s one of the injections, along with rabies, thought to cause cancerous tumors in some cats. Feline leukemia virus is a retrovirus, like HIV in humans, causing white blood cells to grow out of control, which is why it’s called leukemia. This overgrowth of white blood cells leads to a type of cancer called lymphoma.