Abstract: The invention relates to an isolated mammalian negative strand RNA virus, metapneumovirus (MPV), within the sub-family Pneumoviridae, of the family Paramyxoviridae with one or more genetic modifications. The present invention also relates to the mutant components, i.e., nucleic acids and proteins, of these mutant mammalian MPVs. These mutant mMPV can be attenuated. These mutant mMPVs can encode non-native sequences. The invention further relates to vaccine formulations comprising the mMPV, including recombinant and chimeric forms of said viruses. The vaccine preparations of the invention encompass multivalent vaccines, including bivalent and trivalent vaccine preparations. In addition, the invention relates to chimeric viral RNA polymerase complex and assays using these chimeric RNA polymerase complexes. The chimeric RNA polymerase complexes of the invention are composed of different RNA polymerase components from different viruses of the family of paramyxoviridae.