Influenza A virus with attenuating mutations in NS2 protein

An isolated attenuated recombinant influenza virus comprising a gene segment comprising a mutant NS2 protein gene, wherein the NS2 protein has at least one substitution that does not substantially alter the in vitro replication of the virus but is associated with attenuation of the virus in vivo, wherein the at least one substitution is a substitution for glutamate at position 74 or 75 of NS2 relative to position 74 or 75 of NS2 of influenza A virus strain WSN comprising residues in SEQ ID NO:15 which correspond to positions 67 to 82, wherein the at least one substitution is encoded by a mutation in the NS2 gene that also alters the coding region for the NS1 protein thereby yielding a NS1 protein with one or more additional amino acid residues, or wherein the substitution for glutamate is a substitution to a serine, alanine, methionine, glycine, leucine, threonine, isoleucine, or valine.