Phase I/II trial of a replication-deficient trivalent influenza virus vaccine lacking NS1.

•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.