Why do puppies and kittens require a series of vaccinations?
Very young animals are highly susceptible to infectious disease because their immune system is not yet fully mature. They receive protection through antibodies in their mother's milk, but the protection is not long-lasting and there may be gaps in protection as the milk antibodies decrease and their immune system is still maturing.
In many instances, the first dose of a vaccine serves to prime the animal's immune system against the virus or bacteria while subsequent doses help further stimulate the immune system to produce the important antibodies needed to protect an animal from diseases.
To provide optimal protection against disease in the first few months of life, a series of vaccinations are scheduled, usually 3-4 weeks apart. For most puppies and kittens, the final vaccination in the series is administered at about 4 months of age; however, a veterinarian may alter the schedule based on an individual animal's risk factors.
How often will my pet need to be vaccinated?
Many vaccinations provide adequate immunity when administered every few years, while others require more frequent schedules to maintain an acceptable level of immunity that will continually protect your pet. Your veterinarian will determine a vaccination schedule that's appropriate for your pet.
Just like any sort of injection or medication, there is a small risk of reaction.