Most side effects of the flu vaccine are not dangerous at all, as the Centers for Disease Control and Prevention explains. The most common side effects are soreness or redness at the site of the shot or mild nasal congestion if the vaccine is given by nasal spray.
Other mild but less common side effects include muscle aches, headaches and slight fever. In addition, runny nose or sore throat can occur when the vaccine is administered via nasal spray, as Healthline explains. In rare cases, a patient can have an allergic reaction to the flu vaccine. If this occurs, or if a fever of 101 degrees Fahrenheit or higher is experienced, the patient should seek medical attention.