Pain in the arm after vaccination is a normal adverse reaction in most cases and requires no special treatment. If the pain is aggravated and not relieved, you should go to the hospital as soon as possible. 1. Allergic reaction: After the body is exposed to the vaccine components, the immune system will be activated quickly, B lymphocytes and T lymphocytes will be generated in large numbers, and a variety of immune factors will gather at the injection site to form a protective barrier. A variety of immune factors gather at the injection site to form a protective barrier. This process can increase the local metabolism and activate the local tissue hypersensitivity reaction, and the recipient will feel pain and soreness in the arm. This phenomenon is a common adverse reaction, which can be relieved by itself and generally does not require special treatment; 2. Component stimulation: some vaccine components are irritating, and after intramuscular injection, they will directly stimulate local tissues causing pain and soreness, and local hot compresses can be applied to relieve pain symptoms; 3. For example, if a local allergic reaction occurs after vaccination for people with allergies, the symptoms of arm pain will not be relieved continuously without any treatment measures. At this time, anti-allergic drugs such as loratadine and glucocorticoids need to be given as soon as possible; 4. Injection site infection: arm pain caused by infection at the injection site due to improper treatment of skin disinfection during injection or lack of attention to local hygiene after vaccination.