Soluble polypeptide fragment of a RNA-dependent RNA-polymerase PB2 subunit of influenza virus, wherein said soluble polypeptide fragment is selected from the group consisting of: (i) a polypeptide fragment whose N-terminal end is located at the position of amino acid 220 of the PB2 polypeptide according to SEQ ID No. 1 or in an amino acid residue that occupies an analogous position in a PB2 polypeptide aligned therewith or a higher amino acid position； and whose C-terminal end is located at amino acid position 510 of the PB2 polypeptide according to SEQ ID No. 1 or at an amino acid residue that occupies an analogous position in a PB2 polypeptide aligned with it or in a position lower amino acid； or (ii) a polypeptide fragment whose N-terminal end is located at amino acid position 222 of the PB2 polypeptide according to SEQ ID No. 2 or in an amino acid residue occupying an analogous position in an aligned PB2 polypeptide with it or a higher amino acid position； and whose C-terminal end is located at amino acid position 511 of the PB2 polypeptide according to SEQ ID No. 2 or in an amino acid residue that occupies an analogous position in a PB2 polypeptide aligned with it or in a position lower amino acid； or (iii) a polypeptide fragment whose N-terminal end is located at amino acid position 227 of the PB2 polypeptide according to SEQ ID No. 3 or in an amino acid residue occupying an analogous position in an aligned PB2 polypeptide with it or a higher amino acid position； and whose C-terminal end is located at amino acid position 528 of the PB2 polypeptide according to SEQ ID No. 3 or in an amino acid residue that occupies an analogous position in a PB2 polypeptide aligned with it or in a position lower amino acid； or (iv) a variant of the polypeptide fragment, which maintains a sequence identity of at least 80% with the polypeptide fragment conforming to (i), (ii) or (iii) and in which the polypeptide fragment conforming to ( i), (ii) or (iii) or the polypeptide fragm
