The flu vaccine protects you from the influenza virus (type A and B), which causes the flu. The vaccine is given intramuscularly in the deltoid (shoulder) annually, due to its' rapid rate of mutation.
Influenza is a respiratory illness that, if not treated quickly, has the potential to cause significant harm, particularly in high risk inidiviuals such as, young children, the elderly (65 years+), and those with specific health conditions (ie: diabeties, heart disease, obesity etc). The best way to avoid the flu is to get vaccinated in order to help prevent getting the flu and its' complications.
https://www.cdc.gov/flu/prevent/keyfacts.htm
Influenza mutates rapidly and the vaccine you received last year may not protect you against this year’s strains (ie: H1N1). To keep pace with fast-adapting flu viruses, new flu vaccines are developed every year.
Your immune system creates antibodies after being vaccinated creating an active, acquired immunity to the various predicted strains of influenza for the season.
Ideally everyone should get the influenza vaccine, but especially high risk indivdiuals due to increased risk of complications. The Canadian Health Ministry recommends annual flu vaccinations for everyone age 6 months or older. People at high risk of influenza-related problems, such as pregnant women and seniors, are recommended to get immunized.
Children between 6 months and 8 years old might require two doses of the flu vaccine, given four weeks apart, the first time they are vaccinated. Then, after one year, children may receive a single annual injection of the flu vaccine. According to a 2017 study, vaccination significantly lowers a child’s risk of dying from the flu. Consult your child’s doctor if you have any questions.