Studies in children and adults revealed cold–adapted, live, attenuated, trivalent, intranasal influenza vaccine (CAIV–T) to be well accepted, well tolerated and highly protective against culture–confirmed influenza, and to provide significant health benefits. A 2 year, multicentre, double–blind, placebo–controlled efficacy field trial of CAIV–T in children aged 15–71 months with annual re–immunization revealed the vaccine to be highly protective against culture–confirmed influenza. Vaccine induced serum and secretory antibodies in vaccinated children. Overall, during 2 years of study, vaccine was 92% protective against culture–confirmed influenza. During the second year of study the vaccine was 86% protective against influenza A/Sydney/5/97–like virus, a significantly drifted strain not well matched to the vaccine. Antibody studies on children given CAIV–T revealed that high titres of cross–reacting antibodies to influenza A/Sydney/5/97 were induced with vaccination by live attenuated influenza A/Wuhan/359/95–like vaccine. Effectiveness measures revealed significant reductions in febrile illness (21% reduction in year 1, 19% reduction in year 2), febrile otitis media (33% reduction in year 1, 16% reduction in year 2) and associated antibiotic use among vaccinated children compared with placebo recipients. In adults, vaccination with CAIV–T resulted in protection during experimental challenge with virulent wild–type viruses. An effectiveness trial in adults demonstrated significant benefits of CAIV–T vaccine (28% reduction in days of missed work for febrile upper respiratory illness days with associated 45% reduction in days taking antibiotics). General use of CAIV–T has the potential to significantly reduce the impact of influenza in children and adults.