A hard, itchy bump on the arm after the third vaccination may be an adverse reaction to the vaccine. If it is accompanied by redness and swelling, it may also be an infection, which needs to be treated under the guidance of a doctor. The details are as follows: 1. Adverse reaction: after the third injection of the vaccine, when hard lumps and itching appear on the arm, you can use an ice pack to relieve the local symptoms of hard lumps and itching, and gradually reduce the sense of discomfort. After more than 24 hours, use hot compresses to relieve the hard lumps after vaccination, and the hard lumps will gradually dissipate. 2. Infection: If cold or hot compresses cannot relieve the hard lumps and itchy symptoms, accompanied by redness and swelling, consider the local infection, you can first use iodophor to disinfect, and then apply antibiotic ointment under the guidance of the doctor, such as erythromycin ointment, mupirocin ointment and so on. If the above treatment options do not solve the problem of red, swollen, hard and itchy arms, it is recommended that the patient go to the hospital in a timely manner and follow the doctor’s instructions to avoid further deterioration of the condition.