Redness and swelling after vaccination may be caused by adverse reactions to vaccines, contamination of the injection site and improper injection techniques. 1. Adverse vaccine reactions: vaccines are made with pathogens or biological products, and prevent various diseases by making the body produce specific immunity. The immune response triggered by vaccines injected into the body may cause fever, redness and swelling, and other adverse reactions. 2. Contamination of the injection site: when the injection site is not completely scabbed and healed, it may come into contact with external bacteria, resulting in wound infection and thus redness and swelling after vaccination. 3. Improper injection technique: If the injection is too shallow, the dose is too large, or the vaccine is not shaken well, redness, swelling and lumps may appear at the injection site. If the patient has redness and swelling after vaccination, it is recommended to go to the hospital in time to find out the cause and actively cooperate with the doctor for appropriate treatment.