1. Instituto Nacional de Higiene, Epidemiología y MicrobiologíaInfanta No 1158 e/ Llinás y ClavelCódigo Postal 10300La Habana Cuba
2. Nottingham Biomedical Research Unit, Division of Epidemiology and Public HealthUniversity of Nottingham, Clinical Sciences Building, City HospitalNottinghamNG5 1PB United Kingdom