[1] Johns Hopkins Univ, Bloomberg Sch Publ Hlth, Dept Epidemiol, Baltimore, MD 21205 USA
[2] Univ KwaZulu Natal, Nelson R Mandela Sch Med, Dept Paediat & Child Hlth, Durban, South Africa
[3] Univ KwaZulu Natal, Nelson R Mandela Sch Med, Doris Duke Med Res Inst, Durban, South Africa
[4] Greys Hosp, Dept Paediat, Pietermaritzburg, South Africa
[5] Johns Hopkins Univ, Bloomberg Sch Publ Hlth, Dept Int Hlth, Baltimore, MD 21205 USA