The flu shot will protect against the most severe consequences of the flew (Influenza). People of 60 years and older, people of all ages with cardiovascular disease, pulmonary diseases, diabetes, kidney disease, and people with low immune system, have a higher risk to become ill by the flu. When you have one of the listed conditions, you will qualify for a flu shot. You will receive a written invitation. After the flu shot, the chance to be infected by the flu is smaller. When you do get the flu, the symptoms are less severe. The shot must be repeated every year; the best period to get the shot is between half October and half November. The shot can sometimes give a mild pain in the arm for a day, but you will not become ill.
For more information you can check the website of the RIVM