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.