The injection (flu shot) vaccine. Flu vaccine is an inactivated vaccine, meaning that it contains killed influenza virus. The killed influenza virus is injected into muscles and stimulates the immune system to produce an immune response (antibodies) to the influenza virus.he vaccine is termed an attenuated vaccine because the viruses are weakened so that they do not cause severe flu symptoms. The nasal spray flu vaccine (LAIV) has been approved by the FDA for use in nonpregnant healthy people between two and 49 years of age.