If the arm of a vaccination is swollen and a hard lump appears, you need to analyze the causes of arm swelling and hard lump first, and then take targeted treatment measures. In general, the causes and treatments of arm swelling and hard lumps are as follows: 1. Improper injection operation: It leads to the stimulation of local tissues by the medicinal solution, resulting in edema and hard lumps. This can be relieved by hot compresses, but be careful not to scratch with your hands to avoid infection; 2, failure to absorb the injection in time: swelling and hard lumps will appear due to untimely absorption of the injection, and if not accompanied by other abnormal reactions, local hot compresses can be applied to promote absorption; 3, secondary bacterial infection: premature bathing or strenuous exercise, and contact of the injection site with bath water or sweat, causing bacterial infection and arm swelling and hard lumps, which are usually accompanied by increased skin temperature, pain, local skin redness. In this case, it is necessary to go to the hospital in time, and apply antibacterial drugs for treatment after the secondary bacterial infection is clarified; 4. Improper diet: after the vaccination is completed, spicy and stimulating food is eaten, resulting in vasodilation of the injection site, increased vascular permeability, serious local exudation, and arm swelling and hard lumps. It is necessary to strictly avoid spicy and irritating food, such as avoiding drinking alcohol and eating chili, onion, ginger and garlic. At the beginning, you can use ice or ice towel to apply cold compress locally, which helps capillary constriction and reduce exudation, and after 24 hours, you can apply hot compress to promote blood circulation and facilitate absorption of hard lumps; 5. Allergic reaction: due to allergic reaction of vaccine, it may also be that the arm on the vaccination side is exposed to a certain substance that causes allergy, which is usually accompanied by rash and itching. The patient is advised to go to the hospital and use anti-allergic drugs as prescribed by the doctor, such as applying glyburide lotion and taking oral loratadine and paracetamol for treatment.