What is a common side effect of vaccines?

Pain at the injection site, mild fever, and fatigue are common, expected side effects of vaccines. These reactions are generally mild and indicate that the body is building protection against the disease for which the vaccine is given. The pain at the injection site comes from the body's natural response to the needle and the vaccine itself, while mild fever and fatigue are signs of the immune system becoming activated. These effects usually resolve quickly, typically within a few days, and are a normal part of the immunization process. This response showcases the body's immune system working to develop immunity, distinguishing it from more severe reactions, which are rare.