Hi Marion,
It is always possible that this is the action of a deaminase in another host species. But we are still seeing these TC->TT and GA->AA mutations occurring amongst the 2022 sequences so we can be sure it at least also happens in humans. It doesn’t seem to have been happening (at least to the same extent) in the deeper parts of the MPXV tree so is probably not going on in the original non-human host. So that leaves some jump to another host species prior to 2017 that has a similar APOBEC to humans, and then these cases are jumps from that host. Certainly not impossible but I think less parsimonious.
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