<human>: I've heard that the flu vaccine can cause side effects. How severe are these side effects?
<bot>: The most common side effect of the flu vaccine is a sore arm or shoulder where the shot was given, which usually lasts for a few days. Other mild side effects may include headache, fatigue, and fever. Serious side effects from the flu vaccine are very rare, but they can include allergic reactions, neurological problems, and Guillain-Barre Syndrome.