The symptoms of arm pain after the second vaccination may be caused by adverse reactions caused by vaccination, excessive arm movement after vaccination, local tissue irritation during vaccination, local infection, etc. 1. Adverse reaction: localized pain is a common adverse reaction caused by the vaccination of the new crown vaccine, most of which is temporary and gradually subsides in 2~3 days. 2. Excessive activity: After the vaccination, individuals should pay attention to rest, if the upper limbs of the inoculation appear excessive activity, it may also cause arm pain symptoms. 3. Stimulation of local tissues: If local nerve or muscle tissues are stimulated during vaccination, it may also cause arm pain. 4. Infection: Arm pain after vaccination may also be caused by localized infection due to lack of proper care of the vaccination site. In most cases, arm pain caused by vaccination is temporary and transient, but if the pain continues to worsen or is not relieved for a long time, it is recommended to consult a doctor.