Swelling of the arm after the third vaccination may be due to adverse reactions, improper injection practices, and the occurrence of infections. Usually, general treatment and medication can be used. 1. Adverse reaction: If the arm swelling is not very serious, the first thing to consider is the adverse reaction after vaccination, which is a normal phenomenon. Do not need special treatment, usually in 2-3 days or so will gradually disappear, pay attention to do not scratch, in order to avoid secondary infection. 2. Improper injection operation: the third injection of vaccine arm swelling may be due to the improper operation of the injection, resulting in slow absorption of the liquid, stimulation of local tissues, arm swelling. You can promote the local metabolism through hot compresses to promote absorption, thereby alleviating the third vaccination arm swelling, also pay attention not to scratch with your hands to avoid infection. 3. Infection: the third injection of vaccine arm swelling may be due to this premature bath or strenuous exercise, can lead to inoculation in contact with bath water or sweat and lead to infection, resulting in arm swelling. You can apply magnesium sulfate topical application, through the local topical application, will produce osmotic pressure between the local skin, so as to eliminate the third injection of vaccination arm swelling. If the patient has a swollen arm from the third injection of vaccination, it is recommended to go to the hospital in time to find out the cause and then targeted treatment.