Vaccines are continually monitored for safety, and like any medication, vaccines can cause side effects. The overwhelming majority of vaccine-related side effects are mild and short-lived. The risk associated with these side effects is significantly lower than the risk of having severe illness or death from the disease the vaccine is designed to prevent. The most commonly reported side effects include: Redness, mild tenderness or bruising around the injection site; mild muscle pain; low-grade fever; mild irritability; nausea, and fatigue.