[1] Univ San Francisco Quito, Colegio Ciencias Salud, Quito, Ecuador
[2] Univ Fed Bahia, Inst Saude Colet, Salvador, BA, Brazil
[3] Univ Virginia, Asthma & Allerg Dis Ctr, Charlottesville, VA 22903 USA
[4] London Sch Hyg & Trop Med, Dept Epidemiol, London WC1, England
[5] Univ Liverpool, Liverpool Sch Trop Med, Liverpool L3 5QA, Merseyside, England