Redness, swelling and hard lumps on the arm after a booster shot may be caused by the drug not being absorbed, or an irritation reaction and other factors. Redness, swelling and hardness at the injection site after booster injection may be caused by the vaccine not being fully absorbed after injection, which may lead to localized hardness. In addition, poor absorption of the drug may cause local irritation and adverse reactions, resulting in localized redness, swelling and hardness. In addition to this, there are more types of vaccines, if the quality of the vaccine is not good, it may lead to redness, swelling and pain at the injection site. The angle of injection, the depth of injection, and the speed of injection also have some relevance. For example, if the injection is too fast and deep, the medicine cannot be absorbed into the blood in time, which may cause the above symptoms. Localized redness, swelling and hard lumps after booster injection are recommended to pay attention to, and need to give timely treatment, such as infrared physical therapy, to avoid serious consequences.