A new vaccine is developed each year, specific to the three virus strands identified by the Centers for Disease Control (CDC) as most prevalent. These strands will account for 70-90% of influenza that year.
Since the strands of the virus change yearly, you must be vaccinated every year.
The vaccine is created from an inactive form of the virus; you cannot get the flu from a flu shot!!! Common side effects include soreness, swelling, redness at the injection site, low-grade fever, and aches.
Severe reactions are extremely rare. Flu shots are one of the safest, most effective vaccines.