Swelling of the arm after vaccination is usually an adverse reaction to vaccination, and common symptoms include redness and pain in the injection area. In addition to this, the swelling may also be caused by the patient’s injection needle hole being infected by external bacteria, or improper injection technique, muscle tension, and other reasons. Common causes are as follows: 1. Adverse reactions to vaccines: Vaccines are made using pathogens or microorganisms to prevent various diseases by causing the body to produce specific immunity, and the immune response triggered by vaccine injection into the body may cause adverse reactions such as fever and swelling, and the symptoms will be relieved after a few days. If the arm swelling is serious, it may be allergic to the vaccine and induce serious adverse reactions, so it is recommended to seek medical attention promptly; 2. contamination of the injection site: the injection site may come into contact with external bacteria when the scab is not completely healed, resulting in wound infection, so medicine can be used under the guidance of the doctor to eliminate the symptoms of infection and relieve the swelling; 3. improper injection technique: when the injection needle is too shallow, the dose is too large, or the vaccine is not sufficiently shaken, the injection site may become red, swollen and lumpy. Symptoms of redness and swelling at the injection site may occur. If symptoms of fever and pain occur at the same time, feedback and treatment should be provided to the healthcare provider in a timely manner. After vaccination, you should pay attention to keeping the injection site clean and dry, avoid heavy physical work, and avoid alcohol, spicy, stimulating and greasy food. If the vaccination site is swollen and accompanied by severe pain or systemic reactions, such as fever and diarrhea, you should go to the hospital promptly.