Vaccinations are one of the most controversial topics in veterinary medicine, especially in recent years. Vaccinations are intended to stimulate immunity to dangerous viruses or bacteria, so that our dogs and cats are theoretically protected against these pathogens on future exposure to them. For many years, vaccinations have been the major thrust of preventative veterinary medicine; however, in recent years there has been increasing evidence that over vaccination may contribute to many immune mediated diseases, hormonal diseases, neurological disorders and even certain cancers. In my opinion, many available vaccinations have not been shown to be either effective or safe, so many of them I cannot recommend for my patients.
According to a recent edition of Kirk's Current Veterinary Therapy, a text taught in most veterinary schools, immunity to most core viral vaccinations lasts for many years to the life of the pet, just like childhood vaccinations in people. Vaccinating pets more frequently than needed has been shown to carry certain risks, including triggering chronic disease in susceptible pets. It is for these reasons that I have become a minimalist when it comes to most pet vaccinations over my twenty years of experience as a small animal veterinarian.
Probably the most important populations to vaccinate appropriately are puppies and kittens under one year of age. These are the ages that pets are often most susceptible to many viral infections. The core viral vaccinations to use in puppies are canine parvovirus and canine distemper virus (which are often combined into one vaccination). Parvovirus and distemper may cause fatal digestive tract or neurologic disease in young puppies, so it is important to vaccinate puppies appropriately for these two viruses. I do not recommend commonly used 5 or 7 in one vaccinations because of the potential negative and harmful effects on the developing immune systems of young animals. The core viral vaccinations to use in kittens are feline panleukopenia (known as feline parvovirus), feline rhinotracheitis virus and feline calici virus. This 3 in one vaccination offers protection against fatal intestinal symptoms caused by panleukopenia, as well as the respiratory symptoms associated with these other two viruses.

Vaccinations for cats

Vaccinations for dogs

Vaccinations for aging dogs and cats
When given to pets age 14 to 16 weeks or older, vaccination for these core canine and feline viruses lasts for many years in most cases, although many experts recommend a booster at one year of age. The other core viral vaccine for dogs and cats is rabies. Rabies is usually given over 3 months of age to puppies or kittens and then boostered at one year of age. Adult and senior pets usually do not need these vaccinations more frequently than every 3 years, and probably less frequently with viruses other than rabies. As an alternative to yearly or triennial vaccinations, serum antibody titers may also be measured, which often document protective immunity for 7 years or or longer.
Other vaccinations outlined in the chart above are usually not needed or recommended in most ages of pets for either lack of efficacy or safety reasons. Outdoor young kittens under one year of age are susceptible to feline leukemia virus, which may cause immune suppressive illnesses and even cancer in some cats. I therefore will recommend vaccinating outdoor kittens under one year of age for this virus. Adult and senior cats are much less susceptible to becoming infected with feline leukemia virus, so I do not usually recommend vaccinating them, especially if they are indoor cats.