Vaccines are an important part of your pet’s preventative health care plan. They protect your pet from contagious diseases, some of which are fatal, like rabies or hepatitis. Routine vaccination is an important tool in preventing illness and keeping your dog or cat healthy, even if they are primarily indoor animals. But there is some controversy in the veterinary community over the risks of vaccination and how often your pet should be vaccinated.

Because every pet has different risk factors based on their lifestyle, breed, age, medical history, and current health, only your veterinarian can tell you which vaccines are the right choice for your pet and how often your pet should be receiving them.

How do vaccines work?

A vaccine contains dead or weakened microorganisms that cause disease, such as bacteria or viruses. All microorganisms, dead or alive, are covered in molecules known as antigens. When a vaccine is injected into the body, the immune system will detect these unfamiliar antigens, which alerts the immune system to an intruder in the body. In response, certain immune cells will begin to produce proteins called antibodies, which can either destroy the pathogen or “tag” it for destruction by other cells in the immune system.

After the immune system destroys a pathogen once, immune cells known as memory cells become trained to recognize the intruder’s antigens if it ever infects the body again. The next time the body becomes infected by the pathogen, these memory cells will quickly recognize the intruder and launch a faster, stronger antibody response, giving the immune system the upper hand.

This is where the importance of vaccines comes in. When the immune system encounters an intruder like this for the first time, it can sometimes take days for it to generate enough antibodies to completely wipe out the infection. In some cases where the infectious microorganism is at full strength, this response will come too late and the infection will prove fatal. By administering vaccines, the immune system is safely trained to mount a strong, timely response to protect the body from dangerous diseases.

Unfortunately, no vaccine is one-hundred percent effective in preventing disease. Even after vaccination, the possibility of contracting a disease still exists–but the risk is much, much lower than prior to the vaccination. This is especially true when all animals in a community are vaccinated, which creates an effect known as herd immunity. So, even if a vaccine isn’t effective on one dog in the neighborhood, if it protects all the other dogs from infection, the chances for disease transmission are still lowered for the unprotected dog.

What vaccines does my pet need?

Core vaccinations are universally recommended vaccinations for dogs and cats. This includes the rabies vaccination for both dogs and cats, vaccinations against distemper, hepatitis, and parvovirus for dogs, and vaccinations against feline viral rhinotracheitis, calicivirus and panleukopenia for cats.

Non-core vaccinations that protect against diseases like kennel cough, Lyme disease, feline leukemia and others may be recommended by your veterinarian based on your pet’s lifestyle, environment, medical history, and other factors.

Should my indoor pet be vaccinated?

If your pet is an indoor animal, it might seem like there’s no chance of them catching an infectious disease. However, pathogens can be transmitted in many ways other than direct contact with an infected animal. For example, your pet sniffing through a screened window could result in the inhalation of an airborne virus. All sorts of pathogens, from parasitic eggs to viruses, can be brought into the house on shoes, shopping bags, backpacks, toys bought from the store… basically, any contact with the outside world brings with it the risk of disease transmission.

Because of this, veterinarians still recommend all the core vaccinations for your indoor pet. However, some of the non-core vaccinations might be unnecessary depending on your pet’s lifestyle. Your veterinarian can further advise you on what vaccines are right for your pet.

Can vaccines hurt my pet?

There can be side effects to the immune response generated by vaccines. Mild symptoms like soreness at the site of vaccination, lethargy, and loss of appetite are the most common side effects. Typically, these symptoms last only a short time after the vaccination. You should contact your veterinarian if you notice these symptoms persisting for more than a few days.

In rare cases, more serious symptoms can result from an allergic reaction to the vaccine. Signs of an allergic reaction will appear within hours of the vaccination. Symptoms include vomiting, swelling, difficulty breathing, itching across the body, and collapse. If you notice these symptoms, contact your veterinarian immediately.

Cats have a small chance of developing a cancerous tumor at the site of the vaccination, known as a feline injection-site sarcoma (FISS). These tumors can appear at the site of any injection, not just a vaccine. Some vaccines contain substances known as adjuvants, which are used to stimulate the immune system in specific ways. There is evidence that adjuvants heighten the risk of FISS, so non-adjuvanted vaccines are recommended for cats. Research shows that there is about a 0.005% chance of a cat developing FISS after a vaccination. A small lump under the site of the vaccination is usually a normal sign and disappears within one to two weeks. However, if this lump persists after three weeks or begins to grow in size, schedule a veterinary exam for your cat immediately.

That being said, the risk of developing serious complications due to vaccination is much lower than the risks associated with the diseases that these vaccines protect against. The American Veterinary Medical Association, the American Animal Hospital Association, the American Society for the Prevention of Cruelty to Animals, the American Humane Association, and the Humane Society of the United States all strongly suggest that you vaccinate your pets as recommended by your veterinarian.

How often should my pet be vaccinated?

All vaccines come with label recommendations from the pharmaceutical manufacturer on how often they should be administered. They range from yearly for some vaccinations, to every three years for others. This recommendation is based on how long the typical dog or cat will maintain an effective immunity in their body between vaccinations–that is, a level of immunity that will allow the dog or cat to successfully fight off an infection.

In recent years, there has been controversy about “over-vaccination” in pets, which has raised concerns that some veterinarians vaccinate pets when the animal doesn’t really need it. It’s true that some vaccinations are given while pets still have a protective immunity against a disease. Unfortunately, modern veterinary medicine does not yet have the technology to determine when the immunity to a certain pathogen will lapse in an individual dog or cat’s body. Performing a test known as an serologic titer (also called an antibody titer) can tell us the concentration of antibodies in a dog or cat’s blood, but this can only reveal two things: whether the pet has ever been vaccinated (the test will show more than zero antibodies) and whether the pet has been recently infected (the test will show antibodies over a certain threshold). Any quantity in-between only tells us that the animal is not currently sick and it has been vaccinated at some point during its life. Even though we can make some assumptions about what concentration could be protective, the status of the immune system depends on much more than just the number of antibodies present in the bloodstream, so a titer test cannot definitively measure how effective a response to infection would be, and it certainly cannot tell us when a pet will be due for re-vaccination.

Due to ethical and legal reasons, most veterinarians will recommend the suggested vaccine schedule for your pet, even if your pet is perfectly healthy and could still have a protective level of immunity in their body. Veterinarians do not want to gamble with your pet’s health, especially when the risk factors associated with vaccines are so rare compared to the harm that can be caused by diseases like distemper or parvovirus (see “Can vaccines hurt my pet?”).