After the third injection of the new crown vaccine arm swelling hard pain can take hot compresses to relieve, if continued to aggravate the need to consult a doctor. After the third injection of the new crown vaccine arm swelling hard pain performance, mainly considered for the new crown vaccine caused by the adverse reaction, belong to the category of normal adverse reactions. Usually, it can be cured within 3 days, you can use hot towel to apply warm compresses to promote local blood circulation and speed up the recovery. However, if the localized swelling, hardness and pain are not relieved after self-treatment, or even aggravated, with no sign of self-healing, then you need to consult a doctor in time for blood tests and other tests to clarify whether there is a localized infection at the site of vaccination, and then take symptomatic treatment according to the doctor’s instructions.