Your dog doesn’t need to be vaccinated against every disease out there. So how do you decide what vaccines are best for your favorite four-legged friend?

The first step, says Cynthia Cox, DVM, lead veterinarian at the Massachusetts Society for the Prevention of Cruelty to Animals in Boston, is simple: Take your dog to the vet. Your veterinarian will help you come up with a vaccination plan based on your dog’s age, medical history, environment, travel habits and lifestyle.

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

The vaccines your dog shouldn’t skip are the ones considered vital based on risk of exposure, severity of the disease or transmissibility to people, says the American Society for the Prevention of Cruelty to Animals (ASPCA).These are very effective vaccines that are quite safe and designed to prevent easily transmitted diseases, says Cox. “They are generally diseases extremely difficult to treat and commonly fatal.”

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The most important vaccines

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 (CDC) occur in wild animals like raccoons, skunks, bats and foxes. But these animals may bite a dog.

Rabies is a public health concern, Cox says. The rabies virus infects the central nervous system, causing brain disease and death, according to the CDC. Rabies vaccinations have saved the lives of countless dogs and people, according to the ASPCA.

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

2. Distemper combo. This vaccine protects dogs against several diseases. Your vet will choose the best distemper combo for your dog according to her age and risk for specific diseases. But in general, Cox says the diseases the vaccine protects against are the following:

Distemper: Canine distemper is caused by a very contagious virus, and it’s often fatal, says the American Veterinary Medical Association (AVMA). Puppies and dogs usually become infected through virus particles in the air or from the respiratory secretions of infected dogs.

Parvovirus: Parvo attacks the gastrointestinal system and is very contagious, says the AVMA. It spreads by contact with contaminated dogs, their excrement and their equipment such as bowls, collars and clothing. Treating it can be very expensive, and many dogs die despite intensive treatment.

Adenovirus 2: The canine adenovirus type 2 (CAV-2) vaccine is used to protect against canine hepatitis, an infectious liver disease. Adenoviruses are spread through an infected dog’s respiratory secretions or by contact with contaminated feces or urine, according to Merck Animal Health.

Parainfluenza: Canine parainfluenza virus is one of the viral causes of infectious tracheobronchitis or “kennel cough,” which produces flu-like symptoms. It spreads when a dog inhales the excretions of an infected dog.

Your dog is supposed to get a series of vaccines every 3 to 4 weeks until 16 weeks of age during puppyhood, then another a year later. After that, the vaccine should be given every three years.

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Vaccines for different lifestyles

Your veterinarian may recommend additional vaccines depending on your dog’s exposure risk. The most common are:

  • Bordetella: This helps the parainfluenza vaccine prevent “kennel cough.” It’s commonly recommended for dogs who are boarded or go to parks, groomers, play dates and daycare.
  • Leptospirosis: This is generally recommended for dogs who spend time in wooded, swampy areas and who may drink from puddles or swim at ponds accessible to cattle or wildlife.
  • Lyme: This is recommended for dogs who live in warm climates and certain wooded areas of the Northeast, where ticks that carry Lyme disease are prominent, the ASPCA says.
  • Influenza vaccine: Canine influenza virus is “a relatively new disease in dogs,” according to the AVMA, producing flu-like symptoms. [VIN] It’s highly contagious and spreads through respiratory secretions and contaminated objects, like bowls, collars, leashes and surfaces. According to the ASPCA, dogs who come into contact with many other dogs face an increased risk.

Possible vaccine risks

In most cases, the risks from a vaccine are much smaller than the risks of catching the disease.Some things may go wrong, though.

  • No effect. There’s a small chance your dog may still get sick even if vaccinated, according to Cox. His immune system may have been battling another illness at the time of vaccination and couldn’t mount a proper response to the vaccine. Also, some vaccines don't completely prevent a disease, even if everything else is optimal, says Cox.
  • Adverse reaction. Most dogs show no ill effects from vaccination, but some dogs may have a reaction with mild to severe symptoms, according to the ASPCA. Talk to your veterinarian about your dog’s medical history before vaccinating, advises Cox.

After the vaccination, the ASPCA says to look for:

  • Fever
  • Sluggishness
  • Loss of appetite
  • Facial swelling and/or hives
  • Vomiting
  • Diarrhea
  • Pain, swelling, redness, scabbing or hair loss around the injection site
  • Lameness
  • Collapse
  • Difficulty breathing
  • Seizures

Most reactions are mild and will occur right after the vaccination, says Cox, so generally the pet is still at the vet clinic when symptoms show up. If you suspect your dog is having a reaction and you left the vet’s office already, “call your veterinarian and go back.” To be on the safe side, schedule your dog’s appointments so you can monitor him afterward for a few hours.

Don’t administer any vaccines yourself, advises Cox. You may do more harm than good: The injection may not go in because your dog is too furry, you may vaccinate your dog when he is sick or, if your dog is pregnant, you may injure the puppies.

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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.