We all know that Zika has circulated in Africa in the past and possibly conferred immunity in the populations of these settings. Except to that, I will look for having additional insights from all of you.
Although I am not a virology, but I can hypothesize that Africans might have developed some immunity against the virus in a natural way. I am using an analogy of influenza. The efforts being made to develop vaccines in this sense needs to be supported.
I suspect there are three factors.
1) The immunity factor. If women develop immunity as children, they can protect the fetuses they carry just like rubella and many other diseases.
2) Viral drift to a strain that either transmits easier (as an STD) or has more pathology (microcephaly??) See http://phys.org/news/2016-04-genetic-evolution-zika-virus.html for some of the genetic differences.
3) More effective human surveillance. This is a disease with a majority of asymptomatic individuals. Babies with microcephaly months later. Without advanced testing and surveillance, who would know?
We should not be mistaken; ZIKV poses a threat to everyone, including Africans. Unless a country conducts effective surveillance of ZIKV (and related complications), there is no objective way to assess Zika's impact on its people. Please see http://apps.who.int/iris/bitstream/10665/204478/1/zika_2_2016.pdf
http://apps.who.int/iris/bitstream/10665/204478/1/zika_2_2016.pdf
There were indeed genetic drifts while ZKV migrated east, conducing to new viral properties, making easy to genotipically distinguish "african" from "american" ZKV. These new properties allied to a recent introduction in a genetically different human population may explain some of the ZKV impact in American public health.
However I suspect most of this impact is due to improved and more effective surveillance of a new disease with rare but catastrophic outcomes: microcephaly in newborns and Guillain-Barré syndrome, as Dr. Dato commented. In Africa, specially in the area where ZKV is endemic, there are several vector borne diseases with similar clinical pictures and no technical means to diagnose them. In top of all this, up to 80% of infected people have no symptoms at all.
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