The arm pain in the third injection of the new crown vaccine is generally considered to be caused by the vaccination technique, pushing the medicine too fast, and the adverse reaction of the vaccine. 1. Physician’s vaccination technique: each physician’s vaccination technique is a bit different, when the vaccine is injected by different physicians, due to the physician’s vaccination technique is different, which can cause arm pain. 2. Pushing the medicine too fast: If the physician pushes the medicine too fast in order to speed up the progress of vaccination, it can cause arm pain, most of the arm pain will disappear naturally in a few minutes or hours. 3. Vaccine adverse reactions: the new crown vaccine adverse reactions vary from person to person, some people do not have any adverse reactions to the new crown vaccine, and some people can have local pain, itching, fever and other adverse reactions after vaccination. Most of the pain after vaccination will disappear naturally in 1~2 days, if it continues to be unrelieved, you need to go to the hospital for treatment.