In a surprising twist in the decades-long effort to eradicate the virus, authorities in Jerusalem, New York and London have discovered evidence that polio is spreading there, as reported on Sunday.
The original source of the virus? The oral vaccine itself.
Scientists have long known about this extremely rare phenomenon. That is why some countries have switched to other polio vaccines.
But these incidental infections from the oral formula are becoming more glaring as the world inches closer to the eradication of the disease and the number of polio cases caused by the wild, or naturally circulating, virus plummets.