Is the yearly flu vaccine truly and definitely 100% safe for everyone without any shadow of a doubt? i'm scared of the flu but also scared of the shot

Nothing is 100% safe: But literally hundreds of millions of people have received the flu vaccine over the years. There can be side effects, most commonly a little soreness in the arm. You should be more frightened of the flu than the shot. I guarantee that if you get the flu, you will be miserable for a couple weeks. I'll take the sore arm.

Always risk: There will always be some risk with any medication, vaccination, supplement, treatment or procedure. Vaccines are carefully studied for safety and efficacy before being recommended for routine use. Often times, the risk of severe illness or death from the disease being vaccinated against far outweighs the small risk of adverse event from receiving it. It's a personal decision. http://www.fda.gov/BiologicsBloodVaccines/Vaccines/