Red, swollen, hard and itchy arms after the third vaccination are due to the occurrence of adverse vaccine reactions. Common vaccines that require three doses include, hepatitis B vaccine, some of the new crown vaccines, HPV vaccine, etc. Local adverse reactions to vaccines may occur after these vaccinations. Redness, swelling, hardness, and itching of the arm after the third vaccination is normal and is a local reaction after normal vaccination, which is repeatedly informed by medical staff before and during the vaccination. The redness, swelling, hardness and itchiness of the arm after vaccination are due to the fact that the active ingredients and excipients of the vaccine are heterogeneous substances, which can stimulate the immune system of the human body to produce an immune response after entering the body. When there is redness, swelling, hardness and itching of the arm, you can use an ice pack to relieve the local redness, swelling and itching and reduce discomfort. After 24 hours, the ice pack should be replaced by a hot pack. The hot pack can relieve the hard lumps after vaccination and assist in their gradual dissipation. However, care should be taken to avoid getting water in the eye of the needle, as water in the eye of the needle can easily cause infection and aggravate the local redness, swelling and pain. In addition to adverse reactions to vaccines, local allergies or infections may occur at the vaccination site, for example, the patient may be allergic to the rubber gloves of the vaccinating medical staff, or bacteria may enter the body through the needle wound after vaccination, causing redness, swelling, hard lumps and itching. Usually, these are mild cases that need not be worried and can be relieved in a few days. In short, if discomfort still exists after the above treatment, it is recommended to consult a doctor and follow medical advice to avoid aggravation of the condition.