The swelling and itching of the arm after the third vaccination may be related to the following situations: 1. Normal adverse reaction: If the arm swelling is not very serious, it is generally considered to be an adverse reaction after vaccination, which is a normal phenomenon. No special treatment is needed, so be careful not to scratch to avoid secondary infection; 2. Improper injection operation: If such a situation did not occur in the previous vaccination, this vaccination may have improper injection operation, resulting in slow absorption of the medicine, stimulating local tissues and causing redness, swelling and itching. The symptoms can be relieved by means of hot compresses, and the same attention should be paid to not scratching with hands to avoid infection; 3. premature bathing or strenuous exercise: premature bathing or strenuous exercise after vaccination can lead to infection of the inoculation site by contacting bath water or sweat, resulting in swelling and itching, and there may also be symptoms such as redness of the inoculation site, high local skin temperature and pain. Attention should be paid to rest, keep the local clean and dry, and also apply magnesium sulfate externally; 4. eating spicy and stimulating food: eating spicy and stimulating food after the completion of vaccination, such as drinking alcohol and eating chili, onion, ginger, garlic, etc., leads to vasodilation and increased exudation at the vaccination site, resulting in edema and itching. You should drink more water and eat lightly. You can also apply raw potato slices on the red swollen area; 5. Allergic reaction: the patient has recently had irregular work and rest, poor physique or allergic constitution, and has an allergic reaction after vaccination, manifesting as angioneurotic edema with itching. It is also possible that the arm on the vaccination side was exposed to a substance that caused allergy. If this is the case, it is necessary to apply anti-allergic drugs such as loratadine, paracetamol, etc. If the patient cannot determine the cause of the swelling and itchiness of the arm by himself, he needs to go to the hospital in time for the doctor to help analyze and determine the cause. If medication is needed for treatment, it needs to be administered according to the doctor’s prescription.