Cedric has a low fever and minor aches. Yesterday, he went to the doctor for his booster shot and received the flu shot. Which best explains why he has this reaction?

A vaccine can trigger the immune system as it prepares to recognize and fight the flu virus. That immune response may cause mild, short-lived symptoms such as a low fever and aches. So the best explanation is that the vaccine stimulated Cedric’s immune response; an antibiotic or antibodies in the vaccine do not explain this reaction.

AI-written and checked against the verified answer.