Fever 3 days after vaccination is unlikely to be considered a vaccine side effect, and it is important to consider whether the fever is caused by something else that may not be obviously related to the vaccination. Vaccine side effects usually occur on the same day or on the 2nd day after the vaccine is given. The fever is usually mild and does not last very long and can resolve on its own. If the fever occurs 3 days after the vaccination, it is important to consider whether there is a respiratory tract infection or gastrointestinal tract infection that is causing the fever. Observe the combination of symptoms. If there is runny nose, cough, sputum, etc., consider that it is caused by a cold, etc. If there is diarrhea and loss of appetite, consider whether it is caused by a gastrointestinal tract infection.