1. Polio Eradication Programme, World Health Organization Region Office for Africa, Djoue, Brazzaville P.O. Box 06, Congo
2. Vaccine-Preventable Diseases Programme, World Health Organization Regional Office for Africa, Djoue, Brazzaville P.O. Box 06, Congo
3. Department of Community Medicine, Faculty of Clinical Sciences, Bayero University Kano, Zaria Road, Kano P.M.B 3011, Kano State, Nigeria
4. Department of Community Medicine, Aminu Kano Teaching Hospital, Zaria Road, Kano P.M.B 3452, Kano State, Nigeria
5. Cochrane South Africa, South African Medical Research Council, Francie van Zijl Drive, Parrow Valley, Cape Town 7500, South Africa