Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. The Influenza vaccine, also known as a Flu Shot, is an annual vaccine to protect against the highly variable influenza virus. Each injected seasonal influenza vaccine contains three influenza viruses: one A virus, one regular seasonal A virus, and one B virus. "Influenza vaccination is the most effective method for preventing influenza virus infection and its potentially severe complications." An influenza epidemic emerges during each winter''s flu season. Each year there are two flu seasons due to the occurrence of influenza at different times in the Northern and Southern Hemispheres.