can I be forced to have a flu shot?

You can't be forced, however, your employer has the right to ask you to get a flu shot.  Are you going to refuse?   If you do, what do you think their reaction is going to be?   And, if they fire you for not getting a flu shot, I do not believe that you have a lawsuit.   You termination would come under the heading of employment at will. www.njtlaw.com