Clinical and epidemiological evaluation of a live cold-adapted influenza vaccine for 3-14-year-olds.

Considerable progress has been made in recent years in developing live attenuated influenza vaccines with most attention having been directed to the development of cold-adapted (CA) attenuated reassortant vaccines. Such vaccines are widely used in the Russian Federation to immunize children. Findings are reported from a study of live CA reassortant mono- di- and trivalent influenza type A and B vaccines in a series of controlled clinical and epidemiological investigations involving almost 130000 children aged 3-15 years. Clinical immunological and morbidity investigations of the vaccinated children and a control group over a six-month follow-up period indicate that the vaccines were completely attenuated by the children. Transient febrile reactions occurred in less than 1% of the children after vaccination including double seronegative individuals with low antibody titres. The type A reisolates examined were genetically stable. Furthermore the reassortants did not suppress each other after simultaneous inoculation of children and stimulated antibody response to influenza virus strains A1 A3 and B. The incidence of influenza-like diseases was approximately 30-40% lower among the vaccinated group than among the control group. It is noted that this study demonstrates for the first time the efficacy of CA vaccine against infections caused by influenza B virus.