Displays RNA-directed RNA polymerase activity. The mRNA guanylyl transferase and mRNA (guanine-N(7)-)-methyltransferase activities are supposedly provided by cellular enzymes (PubMed:22138959). The template is composed of the viral RNA tightly encapsidated by the nucleoprotein (N). Functions either as transcriptase or as replicase. The transcriptase synthesizes subsequently three subgenomic RNAs, assuring their capping and polyadenylation by a stuttering mechanism. In replicase mode, the polymerase replicates the whole viral genome without recognizing the transcriptional signals. a ribonucleoside 5'-triphosphate + RNA(n) = diphosphate + RNA(n+1) Interacts with the P protein.