[1] Univ Auckland, Dept Gen Practice & Primary Hlth Care, Sch Populat Hlth, Auckland 1, New Zealand
[2] Univ Auckland, Clin Trial Res Unit, Auckland 1, New Zealand
[3] Med Res Inst New Zealand, Wellington, New Zealand
[4] Univ Sydney, George Inst Int Hlth, Sydney, NSW 2006, Australia
[5] Univ Oxford, Clin Trial Serv Unit, Oxford OX1 2JD, England
[6] Univ Oxford, Epidemiol Studies Unit, Oxford OX1 2JD, England