1 University College London, London WC1E 6JB, UK. Electronic address: [email protected].
2 Uganda Virus Research Institute, Entebbe, Uganda.
3 Africa Population and Health Research Centre, Nairobi, Kenya.
4 Centre for Health Research and Training, University of Management and Technology, Freetown, Sierra Leone; School of Community Health Sciences, Njala University, Bo Campus, Bo, Sierra Leone.