Highlights: [•] We performed the first in man study with a live replication-deficient delNS1-trivalent vaccine. [•] Significant antibody titers to influenza strains contained in the vaccine were induced. [•] Seroconversion occurred most frequently in participants who were seronegative before vaccination. [•] A single intranasal dose was well tolerated and safe in healthy male and female participants. [•] No vaccine virus could be re-isolated in volunteers after immunization. [ABSTRACT FROM AUTHOR]