SV40 seroprevalence in two Latin American countries involved in field trials of candidate oral poliovaccines.

•Polyomavirus SV40 was an unrecognized contaminant of early poliovaccines administered to millions.•People in Latin America where vaccine trials took place often have SV40 neutralizing antibodies.•Evidence indicates that SV40 is causing human infections today.•Results of studies of SV40 infections in humans depend on the populations analyzed.