Analysis of risk factors for infant mortality in the 1992-3 and 2002-3 birth cohorts in rural Guinea-Bissau

Abstract

Introduction: Though still high, the infant mortality rate in Guinea-Bissau has declined. We aimed to identify risk factors including vaccination coverage, for infant mortality in the rural population of Guinea-Bissau and assess whether these risk factors changed from 1992-3 to 2002-3.

Methods: The Bandim Health Project (BHP) continuously surveys children in rural Guinea-Bissau. We investigated the association between maternal and infant factors (especially DTP and measles coverage) and infant mortality. Hazard ratios (HR) were calculated using Cox regression. We tested for interactions with sex, age groups (defined by current vaccination schedule) and cohort to assess whether the risk factors were the same for boys and girls, in different age groups in 1992-3 and in 2002-3.

Results: The infant mortality rate declined from 148/1000 person years (PYRS) in 1992-3 to 124/1000 PYRS in 2002-3 (HR = 0.88;95%CI:0.77-0.99); this decline was significant for girls (0.77;0.64-0.94) but not for boys (0.97;0.82-1.15) (p = 0.10 for interaction). Risk factors did not differ significantly by cohort in either distribution or effect. Mortality decline was most marked among girls aged 9-11 months (0.56;0.37-0.83). There was no significant mortality decline for girls 1.5-8 months of age (0.93;0.68-1.28) (p = 0.05 for interaction). DTP and measles coverage increased from 1992-3 to 2002-3.

Conclusions: Risk factors did not change with the decline in mortality. Due to beneficial non-specific effects for girls, the increased coverage of measles vaccination may have contributed to the disproportional decline in mortality by sex and age group.

Fig 1. Vaccination coverage with BCG and DTP in the 1992–3 and 2002–3 birth cohorts among children with a seen vaccination card within 12 months of the assessment age.