Given as one injection in the upper arm, the flu shot contains killed flu viruses that will not cause your child to get the flu, but will cause your child's body to fight off infection by the live flu virus. Getting a shot of the killed virus causes your child to become protected against that particular type of live flu virus if your child comes into contact with it.
A common myth about the flu shot is that it can actually cause the flu. But the flu shot used in the United States is made from killed influenza viruses, which means that it's impossible to catch the flu by getting this vaccine. However, because the nasal spray flu vaccine is made from live viruses, it may cause mild flu-like symptoms, including runny nose, headache, vomiting, muscle aches, and fever.