A mutant protein the amino acid sequence of which comprises a sequence which differs from that of the F protein of a PIV-5 or PIV-2 virus: • by replacement: ∘ of the amino acid which in the sequence for said F protein of the PIV-5 virus, is in position 22, or which, in the sequence for said F protein of the PIV-2 virus, is in position 24, by the amino acid P; and ∘ of the amino acid which in the sequence for said F protein of the PIV-5 virus, is in position 132, or which, in the sequence for said F protein of the PIV-2 virus, is in position 133, by the amino acid E; and ∘ of the amino acid which in the sequence for said F protein of the PIV-5 virus, is in position 290, or which, in the sequence for said F protein of the PIV-2 virus, is in position 294, by the amino acid A; and ∘ of the amino acid which in the sequence for said F protein of the PIV-5 virus, is in position 449, or which, in the sequence for said F protein of the PIV-2 virus, is in position 439, by the amino acid P; • and optionally: o by substitution of the native cleavage site of said F protein by another enzymatic cleavage site, and/or by insertion into said F protein of an enzymatic cleavage site other than the native cleavage site of said F protein; and/or o by deletion of a C-terminal portion of said F protein, said C-terminal portion extending in the N-terminal direction from the last amino acid at the C-terminal end of the protein, but without extending beyond the HR2 domain of said F protein; the sequence for said F protein of the PIV-5 or PIV-2 virus comprising the sequence SEQ ID NO: 34. • by replacement: ∘ of the amino acid which in the sequence for said F protein of the PIV-5 virus, is in position 22, or which, in the sequence for said F protein of the PIV-2 virus, is in position 24, by the amino acid P; and ∘ of the amino acid which in the sequence for said F protein of the PIV-5 virus, is in position 132, or which, in the sequence for said F protein of the PIV-2 virus, is in position 133, by the amino acid E; and ∘ of the amino acid which in the sequence for said F protein of the PIV-5 virus, is in position 290, or which, in the sequence for said F protein of the PIV-2 virus, is in position 294, by the amino acid A; and ∘ of the amino acid which in the sequence for said F protein of the PIV-5 virus, is in position 449, or which, in the sequence for said F protein of the PIV-2 virus, is in position 439, by the amino acid P; ∘ of the amino acid which in the sequence for said F protein of the PIV-5 virus, is in position 22, or which, in the sequence for said F protein of the PIV-2 virus, is in position 24, by the amino acid P; and ∘ of the amino acid which in the sequence for said F protein of the PIV-5 virus, is in position 132, or which, in the sequence for said F protein of the PIV-2 virus, is in position 133, by the amino acid E; and ∘ of the amino acid which in the sequence for said F protein of the PIV-5 virus, is in position 290, or which, in the sequence for said F protein of the PIV-2 virus, is in position 294, by the amino acid A; and ∘ of the amino acid which in the sequence for said F protein of the PIV-5 virus, is in position 449, or which, in the sequence for said F protein of the PIV-2 virus, is in position 439, by the amino acid P; • and optionally: o by substitution of the native cleavage site of said F protein by another enzymatic cleavage site, and/or by insertion into said F protein of an enzymatic cleavage site other than the native cleavage site of said F protein; and/or o by deletion of a C-terminal portion of said F protein, said C-terminal portion extending in the N-terminal direction from the last amino acid at the C-terminal end of the protein, but without extending beyond the HR2 domain of said F protein; o by substitution of the native cleavage site of said F protein by another enzymatic cleavage site, and/or by insertion into said F protein of an enzymatic cleavage site other than the native cleavage site of said F protein; and/or o by deletion of a C-terminal portion of said F protein, said C-terminal portion extending in the N-terminal direction from the last amino acid at the C-terminal end of the protein, but without extending beyond the HR2 domain of said F protein;