The actual evolutionary rate is not actually that important or interesting. The actual value tells you nothing about adaptation - there are too many competing stocastic factors that effect it. Sure, it might be different in a human-mosquito-human cycle than a monkey-mosquito-monkey cycle (or whatever the sylvatic host is) but it doesn’t mean it has changed in nature or is adapting.
The rate is just useful to calibrate the tree to infer the timescale - and getting a good reliable timescale on this outbreak seems like an important goal right now. It is best to calibrate the tree as close to the nodes that you are interested in. To understand when and how it got into South America, what we need is a couple of hundred genomes from affected countries spread over the last five years. They need to be directly sequenced from samples.