1. aLondon School of Hygiene and Tropical Medicine, Keppel Street, London, WC1E 7HT, England.
2. bBiomedical Research and Training Institute, Harare, Zimbabwe.
3. cUniversity College London, London, England.
4. dMinistry of Health and Child Welfare, Harare, Zimbabwe.
5. eMedical Research Council Clinical Trials Unit, London, England.
6. fSouth African Centre for Epidemiological Modelling and Analysis, Stellenbosch, South Africa.