My bad, I was misremembering the paper, which mentions the exonuclease's role in fidelity. I meant RdRps. The conclusion was the same. Figure 4 shows quite clearly that the RdRp mutation is associated with further mutations.

> We found that viral strains with RdRp mutation have a median of 3 point mutations [range: 2–5], whereas viral strains with no RdRp mutation have a median of 1 mutation [range: 0–3] (p value < 0.001, Mann–Whitney test). The different distribution between the two groups relative to the number of mutations is statistically significant (Fig. 4).