Inactivated influenza vaccine has been widely used; however, its effectiveness is not always perfect. To create a much better vaccine, live vaccines have been extensively investigated. Among several candidate live vaccines, cold-adapted(Ca) vaccine is the only promising candidate. According to clinical studies recently conducted in the U.S., Ca vaccine was proven to be highly effective against laboratory confirmed influenza, both in adults and children. Furthermore, Ca vaccines for the H5N1 pandemic strains were generated and their attenuation and efficacy were confirmed in experimental animals.