[1] Harvard Univ, Dept Global Hlth & Populat, Sch Publ Hlth, Boston, MA 02115 USA
[2] Stanford Univ, Dept Med, Prevent Res Ctr, Palo Alto, CA 94304 USA
[3] London Sch Hyg & Trop Med, Dept Hlth Serv Res & Policy, London WC1, England
[4] UCL, Dept Social Epidemiol, London, England
[5] Univ Cambridge, Dept Sociol, Cambridge, England