The hard nodules on the arms after the vaccination are generally considered to be the result of an adverse reaction to the vaccination. The vaccination can help the body to produce protective antibodies, so that the body can have immunity against viral invasion. After receiving the vaccine, some patients may experience some localized adverse reactions, such as hard nodules on the arms. The occurrence of this adverse reaction may be caused by irritation of the local tissues by the vaccine components, or malabsorption of the vaccine components by the local muscle tissues, and the possibility of localized infection cannot be ruled out. Adverse reactions after vaccination are usually mild and temporary, and do not require much concern or special treatment. If the symptoms persist, it is advisable to consult a healthcare professional.