Hello, Anon! Thank you for posting.
Flu vaccines cannot cause flu illness. According to the CDC (center for disease control & prevention), flu vaccines are currently made two ways: the vaccine is made either with a.) flu viruses that have been ‘inactivated’ (killed) and therefore are not infectious, or b.) using only a single gene from a flu virus (as opposed to the full virus) in order to produce an immune response without causing infection.
Some people report having mild reactions to flu vaccination. The most common side effects from flu shots are soreness, redness, tenderness or swelling where the shot was given. Low-grade fever, headache and muscle aches may also occur. If these reactions occur, they usually begin soon after the shot and last 1-2 days. Some people have no side effects at all.