No, a flu shot cannot cause flu illness. Flu vaccines given with a needle are currently made in two ways: the vaccine is made either with a flu vaccine virus that has been 'inactivated' and are therefore not infectious or with no flu vaccine virus at all. The most common side effects from the influenza shot are soreness, redness, tenderness or swelling where the shot was given. Low-grade fever, headache and muscle aches also may occur.
In randomized, blinded studies, where some people get inactivated flu shots and others get salt-water shots, the only differences in symptoms was increased soreness in the arm and redness at the injection site among people who got the flu shot. There were no difference in terms of body aches, fever, cough, runny nose or sore throat.
So, never feel it is better to get the flu than the flu vaccine. Flu can be a serious disease, particularly among young children, older adults, and people with certain chronic health conditions, such as asthma, heart disease or diabetes. Any flu infection can carry a risk of serious complications, hospitalization or death, even among otherwise healthy children and adults.
Getting vaccinated is a safer choice than risking illness to obtain immune protection.