Myocarditis, or inflammation of the heart muscle, can be caused by a number of viruses. When an infection is identified in the body, the immune system triggers an inflammatory response. Similar to a fire alarm, it is a signal to the body that something is wrong. This can be dangerous when it occurs in the heart because it prevents blood from being pumped. The influenza vaccine, however is not delivered to the heart. The immune inflammatory alarm will be triggered either in the muscle or nose where the vaccine is delivered. Swelling at the injection site is a common side effect, but inflammation of the heart is not. The Mayo Clinic promotes vaccination for those with heart conditions, because if you have a heart condition, you are at increased risk of myocarditis following an influenza infection.