Why do some people not feel well after getting the flu shot?
The most common side effect of the flu vaccine in adults is soreness at the spot where the shot was given, which usually lasts less than two days. The soreness is often caused by a person’s immune system making protective antibodies to the killed viruses in the vaccine. These antibodies are what allow the body to fight against flu. The needle stick may also cause some soreness at the injection site. According to the Advisory Committee on Immunization Practices (ACIP), rare symptoms include fever, muscle pain, and feelings of discomfort or weakness. If these problems occur, they are very uncommon and usually begin soon after the shot and last 1-2 days.
