Swollen arms after vaccination may be due to adverse reactions, bacterial infection, allergic reactions and other reasons. 1. Adverse reaction: generally this localized swelling, may be the drug into the human body after the local skin caused by a certain stimulation reaction, so the arm will be swollen and hard phenomenon, generally 1-3 days can be recovered on their own. 2. Bacterial infection: If you touch the water and take a bath within 24 hours after taking the vaccination, it may lead to local infection and the formation of fever, pain, swelling and hardness. 3. Allergic reaction: some people are allergic to the components of the drug, resulting in an allergic reaction after the injection, there will be local skin itching, hard nodules. If the patient has had a preventive injection arm swelling and hardness, you can first observe on your own, such as fever, allergy and other symptoms, it is recommended to go to the hospital in a timely manner, to identify the causes of targeted treatment.