All rabies vaccines administered in the U.S. and Canada are inactivated, or killed, meaning the virus has been processed so it can't cause disease. While most vaccines require two to four initial boosters, the rabies vaccine is a little different. Like other killed vaccines, the initial dose of a rabies vaccine triggers the immune system to be able to create antibodies that can fight rabies if the dog is ever exposed to the virus. Rabies is a slow-acting virus — it can take weeks to months to result in symptoms — which allows a dog's body time to mount an immune response and fight the infection. The rabies vaccine is so effective that it's very rare for dogs who've been vaccinated to become infected.

Vaccine antibodies wane over time, causing the rabies vaccine to lose its efficacy. This is why dogs must get follow-up booster doses. Dogs usually receive one booster a year after their initial shot and then get it once every one to three years to maintain immunity. In most areas, keeping your dog up to date on their rabies vaccine is required by law.

Even though it's rare, a dog can have a severe reaction to the rabies vaccine. This typically isn't because there's something wrong with the vaccine itself, but is due to an overreaction of the dog's immune system.

Serious side effects usually start immediately after or within one hour of vaccination.

Rare reactions to the rabies vaccine include:

• Hives, which appear as firm lumps all over the dog's body and may or may not be itchy
• Vomiting
• Diarrhea
• Swollen face, muzzle and/or eyes
• Severe pain or swelling at the injection site
• Cough
• Collapse or fainting

If you notice any of these signs, immediately take your dog to the veterinarian for emergency treatment.

Loss of energy for a day or two, mild fever, mild soreness and temporary loss of appetite all signal that the vaccine is doing what it's supposed to — stimulating the immune system. If you notice these signs, let your dog rest, shower them with tender, loving care and monitor them for a couple of days. If you're ever worried that your dog might be in pain, call your veterinarian and ask for advice. They may prescribe pain medicine to help your dog feel better.

If you're ever worried, don't hesitate to call your veterinarian. In general, however, it's not necessary to contact the veterinarian unless:

• The mild, expected side effects worsen or continue longer than a few days
• Your dog develops swelling at the injection site that's hot or painful, is weeping, gets larger or doesn't go away after a couple of weeks

Your dog develops any severe or abnormal reactions