The present invention relates to a novel replication deficient influenza virus comprising a modified NS1 segment coding for a NS1 protein lacking a functional RNA binding domain and functional effector domain and having a heterologous sequence inserted between the splice donor site and the splice acceptor site of the NS gene segment. The virus can be used as vector for expression of various proteins like chemokines, cytokines or antigenic structures and to produce vaccines. A fusion peptide comprising part of the N-terminus of an NS1 protein and a signal sequence fused to the C-terminus of said NS1 peptide is also provided.