Arm pain after the third shot of the new coronavirus vaccine may be caused by adverse reactions, localized infections, and so on. 1. Adverse reaction: After the third injection, the vaccine will produce antibodies in the body to prevent the new coronavirus infection. Because the injection site is in the arm, the injection of the new coronavirus vaccine may lead to adverse reactions such as swelling, heat, pain and restriction of movement at the injection site, which can usually be relieved on their own. 2. Localized infection: After the third injection of the new coronavirus vaccine, if you do not pay attention to the cleanliness and hygiene of the injection site, such as contact with pollutants or water, a localized infection may occur, which may also lead to arm pain with redness and swelling, and other symptoms, and you should consult a doctor promptly. After the third vaccination, you should observe for 30 minutes before leaving, so that you can consult the doctor in case of discomfort. Any discomfort should be seen promptly and treated aggressively after the cause is identified under the guidance of the doctor.