Over the past few years, I have written about the importance of vaccinations for pets several times.

It dawned on me while I was explaining the details of vaccinations to a client, I have never written about how vaccinations work, and the reason pets need vaccinations.

A vaccine is a commercially-prepared product that contains the infectious agent in an altered form.

Scientists have been able to take an infectious virus and modify its structure so that it is no longer infectious. The modified virus can be injected into an individual patient causing an immune reaction which protects the patient against the virus without becoming sick.

Immunity is a complicated series of defense mechanisms in which an animal can resist, fight and stop an infection from occurring. The body’s immune system contains cell, such as white blood cells and lymphocytes, which are partly responsible for resisting infection and developing protection against certain organisms.

Like most things involved with medical conditions, nothing 100 percent. Immunity is not always absolutely protective.

That is one of the reasons booster vaccinations and annual vaccinations are necessary.

Most vaccines are administered by injection, which allows the body’s immune system to become exposed and mount a reaction with the result of immunity.

There are vaccines to protect against numerous infections in pets. Several of the infections are combined into one vaccine, requiring only one injection.

One of the more difficult situations with regards to vaccinations to understand is the necessity for puppy booster vaccinations.

A typical vaccine protocol for puppies is to start their vaccines at six weeks old and then every three weeks until they are four months old.

The concept behind this type of vaccine protocol is because of maternal immunity, which is protection from their mother.

Newborn animals have not had a chance to develop their immunity against infectious organisms in their environment. Therefore, they rely on immunity from their mother partly from within the womb and also from the mother’s first milk called colostrum. However, maternal immunity is only temporary. The protection from their mother declines steadily over the first few weeks of life.

Without complicated and costly testing, it is impossible to know precisely when a puppy will lose its maternal immunity. Additionally, any vaccine given while maternal immunity is still present does not work well.

This protocol, with a vaccine given every three weeks, tries to catch the time when maternal immunity is gone, and by administering vaccines so frequently, allows the puppy to react to the vaccine causing immunity to develop.

The rest of pet vaccination protocol is for them to receive vaccinations once a year after the initial puppy and kitten series. The reason for this protocol is because immunity to specific organisms declines over time, especially in older pets. It is imperative for your pet to receive yearly booster vaccinations to maintain their immunity to certain infectious diseases.

Whenever you get a new puppy or kitten, or if your adult pet is not current on their yearly vaccinations, contact your veterinarian as soon as possible to ensure your pet lives a long, healthy and happy life.

Dr. Jeff Castle is a veterinarian at Clark County Veterinary Clinic.