When your dog was first vaccinated, they were given two tiny doses of the bacteria and viruses that cause some common or severe infectious diseases. This stimulates their protective defences and helps build the strength of the immune system. In their first year, this will have helped to protect them from certain illnesses.
Unfortunately this protection doesn't last for their entire life, and in fact gradually diminishes over time.
Booster vaccinations are therefore necessary to maintain this protection for the rest of your dog's life. Not all vaccines require an annual booster, so your vet will ensure that your dog receives the correct injections each year.