There are different ways of administering the flu vaccine as well as different formulations, says the Centers for Disease Control and Prevention. Each year a new flu vaccine is released that is specially formulated against the latest strain of influenza.
The most common flu vaccine contains inactive influenza virus and is injected into the muscle, says WebMD. Other delivery routes of the inactive virus include a nasal spray and an injection into the skin. Traditional flu vaccines protect against three different flu viruses and are called trivalent vaccines, explains the CDC. Quadrivalent vaccines, which protect against four flu viruses, are also available. Vaccines are grown in eggs or in a cell culture. Dose strength varies.