If a vaccine recipient has already received the third shot of vaccine and only the third shot has swollen arms afterwards, it is mostly considered to be caused by the vaccine factor, vaccination method and the patient’s own reasons: 1. The concentration of the booster shot is usually higher, which may lead to muscle pain and swelling after vaccination. The new crown vaccine is a drug and there are inevitably some side effects, if it is a slight muscle pain and swelling, generally no special treatment is needed, only symptomatic treatment is needed, such as hot compresses to promote local absorption of the drug, observe for 2-3 days, if it still does not improve, you must immediately consult a doctor; 2. If the third injection is done with different angles and strengths, it may lead to larger needle holes, more subcutaneous bleeding and slow absorption of drugs, resulting in swelling and pain. Pay attention to not scratching with your hands to avoid infection, observe local symptoms and go to the hospital immediately if there is infection, and take antibiotics according to medical advice; 3. This symptom may also occur due to the presence of diseases such as cold and flu, lower immunity and increased sensitivity to the vaccine; or the vaccinated arm may come into contact with allergic substances and develop allergy, causing swelling of the arm. If the infection is caused by premature bathing, the patient should immediately consult a doctor, conduct a drug sensitivity test, use the appropriate antibiotics, and carry out treatment; if the infection is caused by a cold, there is no need to panic, and the cold can generally be cured. The allergen should be detected and anti-allergy medication should be applied. Usually, mild swelling and pain at the vaccination site are normal phenomena, and there is no need to worry or take measures to relieve them, and they can generally disappear on their own after 2-3 days.