1. Swiss Tropical and Public Health Institute , Kreuzstrasse 2, Allschwil 4123, Switzerland
2. University of Basel , Petersplatz 1, P. O. Box 4001, Basel, Switzerland
3. Centre for Multidisciplinary Research in Health Science (MACH), University of Milan , Via Festa del Perdono, 7, Milano, 20122, Italy
4. Polio Transition Programme, World Health Organization , 20 Avenue Appia, 1211 Geneva, Switzerland