[1] Univ Witwatersrand, Fac Hlth Sci, Dept Paediat & Child Hlth, ESRU,Rahima Moosa Mother & Child Hosp, Johannesburg, South Africa
[2] Columbia Univ, Gertrude H Sergievsky Ctr, Coll Phys & Surg, New York, NY 10027 USA
[3] Columbia Univ, Mailman Sch Publ Hlth, Dept Epidemiol, New York, NY USA
[4] Columbia Univ, Mailman Sch Publ Hlth, ICAP, New York, NY USA