After vaccination, if the baby develops pus in the arm, it should be treated promptly, and if the pus is serious, it may require incision and drainage. It is recommended to use anti-inflammatory drugs for treatment. You can take cefaclor granules orally and apply skin disinfectants locally, such as disinfection with iodophor and deiodination with alcohol, to keep the skin clean. If there is no open wound, you can also apply hot towels externally, which can play the function of dissipating local bruises and swelling, and try not to touch the area with your hands during the treatment period, otherwise it is likely to lead to aggravation of pain and infection symptoms. If the local symptoms are still not relieved by conservative treatment, go to the hospital in time for incision and drainage. If fever occurs during the period, it is likely that the infection is aggravated and timely high doses of antibiotics can be used, and intravenous drips of anti-inflammatory drugs can be chosen for treatment.