f protein of the wr isolate of piv 5 presented in fig1 a or of said alternative sequence u201c seq id no 31 with s at 443 u201d highly preferably in said alternative sequence u201c seq id no 31 with s at 443 u201d said sequence derivable from that of the f protein of piv 5 does not have to comprise a mutation other than the mutations 22p 132e 290a 449p mentioned above with respect to said f protein sequence for piv 5 this is in particular the case with the sequence of seq id no 65 fus8 alternatively said sequence derivable from that of the f protein of piv 5 may be derivable from this f protein sequence by said mutations 22p 132e 290a 449p mentioned above and by at least one mutation other than these mutations 22p 132e 290a 449p mentioned above preferably by replacement of the amino acid in position 49 by the amino acid a replacement of the amino acid in position 402 by the amino acid a replacement of the amino acid in position 443 by the amino acid p replacement of the amino acid in position 447 by the amino acid p replacement of the amino acid in position 147 by a hydrophobic amino acid replacement of the amino acid in.