[1] Univ Malaya, Dept Social & Prevent Med, Kuala Lumpur 50603, Malaysia
[2] Univ Malaya, Dept Paediat, Kuala Lumpur 50603, Malaysia
[3] Univ Malaya, Paediat & Child Hlth Res Grp, Kuala Lumpur 50603, Malaysia
[4] Hosp Sultanah Nur Zahirah, Dept Paediat, Kuala Terengganu, Malaysia
[5] Publ Hlth England, Modeling & Econ Unit, London, England
[6] London Sch Hyg & Trop Med, Dept Infect Dis Epidemiol, London WC1, England