The flu shot can cause mild side effects that are sometimes mistaken for flu. For example, people sometimes experience a sore arm where the shot was given. The soreness is often caused by a person’s immune system making protective antibodies in response to being vaccinated. These antibodies are what allow the body to fight against flu. The needle stick may also cause some soreness at the injection site. Rarely, people who get the flu shot have fever, muscle pain, and feelings of discomfort or weakness. If experienced at all, these effects usually last 1 to 2 days after vaccination and are much less severe than actual flu illness.