1. Medical Research Council Epidemiology Unit, University of Cambridge, Cambridge, United Kingdom.
2. Send correspondence to Ahmed Razavi, ahmed.razavi@mrc-epid.cam.ac.uk
3. George Alleyne Chronic Disease Research Centre, Caribbean Institute for Health Research, University of the West Indies, Bridgetown, Barbados.
4. Faculty of Medical Sciences, University of the West Indies, Bridgetown, Barbados.