It takes only one injection each year to protect you from the flu. The vaccine is very safe. The most common side effect is a sore arm from the injection. Some people may suffer a few hours of fever, muscle pain or chills.
You need to get the vaccine each year because last year's vaccine offers almost no protection whatever. Each year's new strain has a completely different structure than the virus that appeared the previous year. Every year a new flu virus circulates the globe, so in January, a year ahead, a committee decides which new viruses are likely to come around and cause that year's winter epidemic of flu. Then manufacturers of the flu vaccine can use that virus in their vaccines for the next year's flu epidemic.
This year (2018-2019), the vaccine contains a virus that has less than a 10 percent chance of being the cause of this year’s epidemic. Even so, you are better off with the vaccine than without it. Since flu viruses share some surface antigens, you can still get partial protection that may save your life from a virus that may be related to this year's epidemic virus.
The vaccine is particularly important for women who are more than three months pregnant, people with lung disease and the elderly because they are the ones most likely to suffer complications and require hospitalization if they get the flu. Health care workers and others who are exposed to large numbers of people should also get their flu shots every year.