A flu shot cannot cause flu illness. The flu vaccine does not have a live virus in it, so it is impossible to get the flu from the vaccine.
The most common side effects from the flu shot are soreness, redness, tenderness or swelling where the shot was given. Low-grade fever, headache and muscle aches also may occur. These are expected side effects from the vaccine and generally do not last long. These reactions should not be misinterpreted as getting sick with flu from the vaccine. The most common reactions people have to the flu vaccine are considerably less severe than the symptoms caused by actual flu illness.
In randomized, blinded studies, where some adults 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 differences in terms of body aches, fever, cough, runny nose or sore throat.
Serious allergic reactions to flu vaccines are very rare. If they do occur, it is usually within a few minutes to a few hours after the vaccination. If you have questions or concerns about the flu vaccine, be sure to talk to your child’s doctor.
CDC – Misconceptions about Seasonal Flu and Flu Vaccines