Abstract: Provided are live attenuated influenza A and B viruses as well as a composition, influenza A and B genes, a vector with a respective gene, a host cell comprising such vector, a method for preparing a live attenuated influenza A or B virus and a use of the influenza viruses as a vaccine. An influenza A virus contains a NP-gene, which includes a silent mutation at one or more positions selected from nucleotide 1467, nucleotide 1473, nucleotide 1500, nucleotide 1503, nucleotide 1512, nucleotide 1515, nucleotide 1518, nucleotide 1521, and nucleotide 1524 of SEQ ID No: 1. A PA gene includes a silent mutation at one or more positions corresponding to a position selected from nucleotides 2100 and 2103 of SEQ ID No: 3.