If you’re under the impression your indoor cats don’t need vaccinations because they aren’t exposed to the outdoors, think again.

Many pet parents believe their cats can skip vaccines because they never go outside, says Krista Vernaleken, VMD, medical director at Bulger Veterinary Hospital in North Andover, Massachusetts. She admits indoor cats are exposed to little risk compared to outdoor cats. But even if your kitty is restricted to your home, he can still escape or get in touch with critters that may enter the house, such as rodents and bats.

“Have the vet go over with you the specific risk factors that your cat might have,” Vernaleken says. Your kitty will need a vaccination plan based on her age, medical history, environment and lifestyle, according to the American Society for the Prevention of Cruelty to Animals (ASPCA).

Cat vaccines work like human vaccines: They generally contain an agent that resembles a disease-causing microorganism (often, a virus) but doesn’t cause disease. The vaccine prepares your kitty to fight off the real thing if she’s ever exposed to it.

Related: 2 Vaccines Your Dog Absolutely Needs, and 4 He Might

The most important vaccines

The vaccines your cat shouldn’t skip are the ones considered vital based on risk of disease exposure, severity or transmissibility to people, says Cynthia Cox, DVM, lead veterinarian at the Massachusetts Society for the Prevention of Cruelty to Animals in Boston. They are:

1. Rabies. Rabies is a viral disease transmitted most often through a rabid animal’s bite. The majority of rabies cases reported to the Centers for Disease Control and Prevention occur in wild animals like raccoons, skunks, bats and foxes. But these animals may bite your cat, especially if you have an outdoor cat.

The vaccine shouldn’t be given to cats younger than 12 weeks, as kittens have antibodies from their mother that may block the effect of the vaccine if it’s given too soon, says Vernaleken.

2. Distemper combo. This vaccine protects cats against several diseases. Your vet will choose the best combo for your cat. Most times, it will include panleukopenia, rhinotracheitis and calicivirus. In rare occasions, it may contain chlamydia. But that one is rarely given, says Cox, because it’s generally only needed in catteries or shelters with documented outbreaks of the disease.

  • Panleukopenia: Also called “feline distemper,” this vaccine prevents a highly contagious disease caused by the feline parvovirus. That virus produces bloody diarrhea, anemia and shortage of white blood cells. Because this virus is everywhere, virtually all cats are exposed to it at some point. The rates of illness and death are high in unvaccinated cats, says the American Veterinary Medical Association (AVMA). People can’t catch it.
  • Rhinotracheitis: Also known as “feline viral rhinopneumonitis” and “herpesvirus type 1,” this is one of the most common causes of upper respiratory infections in cats. Many cats are exposed to this very contagious virus at some point, whether in catteries, shelters or multi-cat households, according to the ASPCA. It generally spreads through contact with discharge from a cat’s eyes, mouth or nose by sharing litter boxes, food and water bowls and during mutual grooming. Some symptoms are sneezing attacks, nose and eye discharge, conjunctivitis, eye ulcers, congestion, fever and loss of appetite, the ASPCA says. Cats weakened by it may also develop secondary infections. People and dogs aren’t at risk.
  • Calicivirus: It’s one of the more common causes of respiratory infection in cats. There are dozens of different strains, some more potent than others. It causes flu-like symptoms, mouth and nose ulcers, anorexia, lethargy, fever, enlarged lymph nodes, squinting, joints lameness, severe depression, leg and face edema, jaundice, multiple organ disease and even death, the ASPCA says. It’s prevalent in shelters, catteries and multi-cat households and is transmitted through sneezing, coughing, grooming or sharing food and water. Once infected, cats can become carriers for life and can transmit the virus even when not showing clinical signs. The ASPCA says cats often develop secondary bacterial infections.

Your cat is supposed to get a series of distemper combo vaccines every 3 to 4 weeks until 16 weeks of age, then another a year later. After that, the vaccine should be given every three years, according to Cox.

Related: 7 Cat Carrier Mistakes and How to Avoid Them

Another possible vaccine

Feline leukemia. Your vet may recommend this vaccine if your cat goes outside or lives inside but may meet infected or outdoor cats. This virus is one of the most commonly diagnosed causes of disease and death in domestic cats, according to the ASPCA. It doesn’t always cause symptoms right away and can severely hinder the immune system.

It’s most commonly transmitted through contact with bodily fluids, mutual grooming and sharing litter boxes, food and water bowls. It also can be passed in utero or through mother’s milk.

Possible vaccine risks

Most cats show no ill effects from vaccination, and vaccine reactions are usually minor and short-lived, says he ASPCA. Some things may go wrong, though.

  • No effect. There’s a small chance your cat may get sick even if vaccinated, according to Cox. His immune system may have been compromised at the time of vaccination and couldn’t mount a proper response to the vaccine. Also, some vaccines don't completely prevent a disease, says Cox.
  • Side effects. Mild side effects such as local swelling and discomfort at the vaccination site, mild fever, lethargy and decreased appetite may happen within a day or two, Cox says. More serious — and less common — side effects such as persistent vomiting and diarrhea, itchy and bumpy skin, facial swelling, difficulty breathing and collapse generally happen right after vaccination and can be life threatening, Cox warns. If that happens,call your vet and rush back.”
  • Sarcomas. A rare but serious adverse reaction is tumor growth, which can develop weeks, months, or even years after a vaccination. “Reports indicate that they occur at a rate of about 1 case per 10,000 to 30,000 vaccinations,” says the AVMA.

“Vaccines are the easiest and most economical way to prevent severe disease,” says Cox. “And even if your cat is on the three-year vaccine schedule,” she adds, “annual exams are still critical to monitor your cat’s health.”

Related: Introducing a New Cat to Your Home

Daniela Caride is a freelance writer who has four cats and two dogs. She blogs about being a pet parent at Taildom.com and founded a nonprofit called Phinney's Friends.