I think it depends on the person. I know some have had no difficulty. My daughter who has SWCAH, has never done well after a flu shot. She is eight now. She has always ran a fever, and went through a runny nose, cough thing. I just called today to schedule her’s and made sure it is on a Friday because I know for the next few days she is down. As she gets older I think it is less and less of an effect though. We will see.