To prevent certain infectious diseases, children are required to receive vaccinations or oral vaccines, such as DPT3, B encephalitis vaccine, polio pills, etc. In outpatient and emergency clinics, we often see parents who are overwhelmed by their children’s fever after vaccination and make several trips to the hospital a day. In fact, as long as the fever is caused by the vaccination, parents should not worry too much. Because the human body reacts to any external stimuli, preventive vaccines are biological products, and children generally have different degrees of local or systemic reactions after vaccination or oral administration due to the stimulation of attenuated or inactivated vaccine itself and its acidity or alkalinity. Common local reactions are varying degrees of redness, swelling, heat, and pain at the injection site about 24 hours after vaccination. The common systemic reaction is fever. Generally, the anal temperature of children below 38℃ is a weak reaction, 38℃-39℃ is a moderate reaction, and above 39℃ is a strong reaction. Fever can be accompanied by mild or severe headache, chills, nausea, vomiting, abdominal pain, diarrhea and other symptoms. At present, most children have mild local and systemic reactions to the vaccinations promoted in China. Therefore, fever that does not exceed 38.5℃ (anal temperature) after vaccination can be treated without special treatment, paying attention to children’s regular life, proper rest, and drinking more boiled water, which can generally return to normal in 1-2 days. Fever above moderate level after vaccination may be caused by the following conditions: 1. children who are particularly sensitive to vaccine reactions; 2. children with weak constitution or suffering from cold or other diseases before vaccination; 3. children who have already felt the infectious disease and are in the incubation period; 4. excessive dose of vaccine injection. Children with higher fever or longer duration of fever after vaccination should still go to the hospital for treatment in time.