There are several different ways a dog's body protects itself from disease. When puppies are born, they become temporarily protected from disease by nursing from their mother. The first milk she produces, called colostrum, contains maternal antibodies that protect the pup during the first weeks of life. Dogs can also acquire what is called natural, or active, immunity if they become ill with — and survive — a particular infectious disease. The antibodies formed from natural exposure usually last for life.

Vaccination provides what is known as acquired immunity. A vaccine is a substance that, when injected, provides immunity against infectious diseases caused by bacteria, viruses, and other organisms. It does this by challenging the body with modified disease organisms, provoking the immune system to form antibodies against those particular organisms. Vaccinations don't necessarily provide lifelong immunity and must be repeated at certain intervals.

Necessary or Optional?

The decision to vaccinate a dog against a particular disease depends partly on the dog's age, breed, and potential exposure to the disease. It also has a lot to do with the distribution and virulence of the particular disease. Certain canine diseases are widely distributed, highly contagious among dogs, and serious or sometimes even fatal. Once a dog has acquired them, no treatment other than supportive therapy can help. These diseases are canine parvovirus, canine distemper virus, canine adenovirus, and rabies (which besides being fatal is also transmissible to humans, making it a public health threat). The vaccines against these diseases are referred to as core vaccines, meaning that they are recommended for all puppies and dogs.

Other vaccines are considered optional. When deciding whether to give them, you and your veterinarian should take into account the geographic distribution of the disease and your dog's risk of exposure. For instance, if your beagle is a show dog that never sets foot in field or forest, she's not at very high risk for Lyme disease. The non-core vaccines are those against canine para-influenza virus, Bordetella bronchiseptica (for canine cough, also known as kennel cough), leptospirosis, giardiasis, and Lyme disease. The distemper-measles combination is also considered a non-core vaccine.

According to the vaccination guidelines of the Veterinary Medical Teaching Hospital at the University of California at Davis, vaccination with these non-core vaccines is generally less effective in protecting against disease than vaccination with the core vaccines. Some of these diseases are not common in many areas of the country, so there's no reason to give them unless your dog is at risk. Your veterinarian can tell you which are necessary in your area.

When to Vaccinate

Puppies receive vaccinations against parvovirus, distemper, and adeno-virus-2 (canine hepatitis) at six to eight weeks of age, again at nine to eleven weeks, and at twelve to sixteen weeks, for the final series. A dog older than four months of age with an unknown vaccination history needs one dose of vaccine against these diseases. After a booster shot at one year, most authorities recommend revaccination every three years.

The first rabies vaccination is generally given at sixteen weeks (four months) of age. Boosters are usually given at three-year intervals, although a few states require annual rabies vaccination. Adult dogs with an unknown vaccination history need a rabies vaccination as well.

How Often to Vaccinate

Most vaccine labels recommend that the vaccine be administered annually. No scientific studies support this frequency of vaccination, however, and there's good evidence that the effects of vaccination last for much longer. While your dog still needs an annual exam, there's usually no reason for her to receive annual vaccinations. Frequent vaccinations may be advisable under some circumstances, so be sure to speak to your vet. For example, the Bordetella vaccine (which is given nasally) does not appear to last very long. If you kennel your dog frequently, or she often comes in contact with strange dogs at shows or dog parks, she may need this vaccine up to twice a year.

Why wouldn't you want to vaccinate every year? Better safe than sorry, right? Actually, that's not always the case. Too-frequent vaccination has been associated with autoimmune diseases such as autoimmune hemolytic anemia, and many veterinarians and dog owners are concerned that over-vaccination can have other ill effects that are not yet known or understood. Before you schedule your dog for revaccination against distemper, parvovirus, or adenovirus-2, you may want to have her titer levels tested. A titer is the concentration of an antibody in blood serum. If these levels are still high, she probably doesn't need to be revaccinated.

New Developments in Vaccines

A new type of immunization, called a recombinant vaccine, is created by splicing gene-size fragments of DNA from a virus or bacterium. The recombinant vaccines that have been developed so far — for distemper, rabies, and Lyme disease — are safe and effective. They work by delivering specific antigen material to the dog on a cellular level, reducing the risk of vaccine reactions, which sometimes occur with vaccines that contain the entire disease-causing organism. When recombinant vaccines become more widespread, it may be necessary to reconsider the age at which vaccines are first given, as well as the interval between vaccines.