The numbers and rosy picture here aren’t quite as nice as other—more detailed—reporting in the Economist recently would lead us to believe.
In some sense I do appreciate the sophistication of Bill Gates’ science communication here though as I suspect that far more Westerners are his audience and a much larger proportion of them are uninformed anti-vaxxers who might latch onto the idea of vaccine-derived polio cases as further evidence for their worldview of not vaccinating their own children and thereby increasing heath risk in the United States.