Hi, Thanks for writing in. Flu jab can cause side effects in many people, especially in those who have known allergy to other substances including egg protein. Side effects can be mild or moderate. It can range from hives, itching and redness over the area of injection. It can also cause dizziness, fever, severe muscle pain and fatigue. Rarely it can cause anaphylaxis reactions. Please have him seen by a doctor and give him antihistamine like Benadryl. He may be given other antihistamines by the doctor. If he has had a heart attack, please get an ECG done and monitor his pulse for a while. Hope this helps