But it does. It takes about two weeks for your body to develop sufficient antibodies after the flu shot. You can still get sick during that window.
Furthermore, the influenza virus is always changing. Without getting to technical, the membrane, or outer covering of the virus changes annually.
In the Dallas Ft. Worth area, there are several strands floating around.
The people making the flushot tries to predict what strands will be most common each year, so they develop a shot that covers the most general strand out. The most common strand could be 33% or 45% or even 15%, etc. If you come into contact with the strand that the flu shot was designed for, then you'll be gucci, but if you come into contact with different strand, then youre outta luck.
I generally don't take the flu shot myself because I ****ing hate needles, lol, but im obligated to take it because my school and hospital requires me. It's in the interest of the patients, they don't give a **** about me lmfao