Why We Vaccinate Our Dogs and Cats

Pet vaccines don’t just protect our beloved animals—some of them protect us, too.

Vaccines for pets have a lot in common with vaccines for humans, from how they’re developed and tested to the purpose they serve in reducing disease and transmission among animals. But unlike human vaccines, pet vaccines play an additional role: “They’re designed to help protect your pet, and they’re also designed to help protect you in some cases,” explains Meghan Davis, DVM, PhD ’12, MPH ’08, a veterinarian and an associate professor in Environmental Health and Engineering.

Are pet vaccinations legally required?

Most U.S. states have laws that require rabies vaccination for pets, but the laws vary. Some require it for all dogs and cats (and sometimes ferrets), some only for dogs, and some only for pets imported from outside the continental U.S. Some states defer to county health departments to set rabies vaccination requirements. Notably, Hawaii is the only rabies-free state and thus maintains strict vaccination and quarantine laws.

Rabies can spread from animals to humans—and it’s nearly always fatal, Davis emphasizes, so keeping pets up to date on rabies vaccination is essential. If a pet interacts with wildlife or bites or scratches a person or another animal, knowing the pet’s rabies vaccination status is critical in determining what actions may be warranted. A human who is bitten or scratched by an animal without up-to-date rabies vaccination may require post-exposure prophylaxis and vaccination. Animals who are not vaccinated, or whose vaccination status cannot be verified, may have to be quarantined or euthanized.

All other vaccines, while not legally required, are recommended routinely to prevent common infectious diseases. In addition to rabies, the bacterial disease leptospirosis circulates primarily in animals but can be transmitted from animals to humans. Dogs are more likely than humans to come in contact with an infection source, and subsequently risk transmitting it to humans.

Other vaccine-preventable illnesses are highly infectious and carry a higher risk of mortality or significant impact on the pet’s lifespan. “Humans aren’t necessarily at risk, but the pets are at really high risk for things like feline leukemia or parvovirus in dogs,” says Davis. “Parvo puppies are the last thing a veterinarian wants to see, because they’re really difficult ICU cases and they’re highly infectious to other dogs.”

How has rabies vaccination in the U.S. impacted the disease’s prevalence?

“Rabies [vaccination] laws are not in place for animal health. They are in place for human health,” says Davis. Thanks in large part to public health surveillance and successful vaccination of pets over the last several decades, less than 10% of U.S. rabies cases occur in domesticated pets. In that same time, human deaths from rabies have also declined from hundreds per year to less than 10.

The majority of cases are in wildlife, like raccoons, bats, foxes, and skunks, and according to the CDC, three in four Americans live in communities where wildlife rabies presents a risk. “Your dog is more likely to have contact with a raccoon or fox and then become a vector to transmit it to a human,” Davis explains. For the same reason, rabies vaccines are recommended for cats, even those that spend their entire lives indoors: If a bat enters your home, or your cat slips out the front door, the vaccine protects them from getting rabies.

Do all dogs need leptospirosis vaccination?

Leptospirosis is a disease caused by the Leptospira bacteria and circulates commonly among mammals, particularly wildlife and rodents. The bacteria are excreted through an animal’s urine and bodily fluids and can persist in water and soil for weeks to months. “If the dog gets it, the human is also at risk of getting it from the dog,” Davis explains, so vaccinating dogs keeps them from becoming infected and from transmitting it to their humans. Dogs can become infected through contact with the bacteria by doing things like drinking from a contaminated stream or puddle or interacting with an infected animal, or through exposure of a skin wound to infected water or soil.

Until recently, leptospirosis vaccination was recommended only for dogs living in certain regions or who spent a lot of time outdoors. But with rising prevalence of leptospirosis across the U.S., including in urban settings, in 2023, the American College of Veterinary Internal Medicine recommended that all dogs be vaccinated annually for leptospirosis.

Is it safe to vaccinate your pet?

Vaccination for pets is very safe. While there are rare occurrences of severe adverse reactions, the vast majority of pets tolerate them well. “These vaccines go through pretty much identical, but slightly accelerated, testing compared to human vaccines,” Davis says. “You can get through an animal trial faster than a human trial ... but what you do in each trial phase is almost identical.” And, like human vaccines, pet vaccines are monitored for adverse reactions or safety concerns once they’re in use in the general population.

Exposure and infection pose a much greater risk to an animal than vaccination does, says Davis. While vaccines do not provide 100% protection against infection, they significantly reduce the chance of infection. And if a vaccinated dog or cat does get sick, the vaccine reduces the chance of severe symptoms or lasting effects. “Lyme disease, for example, can cause chronic health effects, including kidney disease and polyarthritis,” but the chances are lower and the symptoms milder if a pet is vaccinated, Davis says.

How do I know which vaccines my pet really needs?

Talk to your veterinarian about your pet’s lifestyle and risk factors. Vaccines for pets generally fall into two categories: core vaccines that are universally recommended; and noncore vaccines for dogs, which a vet may recommend depending on a dog’s geographic location or exposure risk.

A dog that spends time at doggy daycare, a grooming or boarding facility, or the dog park will be at higher risk for respiratory viruses than one that does not socialize with other dogs. Those dogs should stay up to date on vaccines for Bordetella bronchiseptica and canine influenza—in fact, many boarding and daycare facilities require it.

For dogs that spend a lot of time outdoors, especially in areas where ticks are present, a vaccine for Lyme disease would likely be recommended, says Davis. On the other hand, for a teacup Chihuahua that only ever leaves the house in its owner’s purse, “we might just stick to the combination distemper vaccine, which also protects against parvovirus, and rabies vaccines.” Vaccinations may be more strongly encouraged for older pets, which can be at higher risk of infection following exposure.

For pets with a history of adverse reactions to vaccination or who cannot be vaccinated due to age or health condition, titer testing can be an option to gauge whether the pet has sufficient immunity from a previous exposure or vaccination. “But there are a limited number of titers you can run, at least in a commercial setting,” Davis adds.

What barriers keep people from vaccinating their pets?

Pet owners in the U.S. have become increasingly hesitant to vaccinate their pets. In some cases, that stems from concerns the owner may have around vaccines for humans. “If people are vaccine-hesitant for themselves and their families, you tend to also see that within pet vaccines,” Davis explains.

Pet vaccine hesitancy can also stem from incomplete or outdated information, says Davis. For example, an owner whose previous cat developed cancer at the site of a vaccine injection may not feel comfortable risking that side effect with a cat they’ve just adopted. Davis says that rare reactions like that do occur, adding that vaccine formulations and protocols have adapted over the years to greatly reduce the likelihood of injection-site carcinomas and improve the ability to treat those that do occur.

On top of that, not everyone can access reliable veterinary care, whether due to cost, distance, lack of transportation, or the pet’s mobility or temperament. Pop-up and mobile vaccine clinics are one way shelters and health departments try to bridge this gap, Davis says, but they don’t offer owners the same care as a routine wellness visit. “A rabies vaccination clinic is a compliance mechanism to provide low-cost services for people who need it,” she says. Wellness exams, on the other hand, give veterinarians the opportunity to tailor preventive care, catch and treat illnesses early, and importantly, address owners’ questions or concerns about their pets’ health.