Comparative analysis of patterns of survival by season of birth in rural Bangladeshi and Gambian populations.

BACKGROUND: Analysis of data from rural Gambia has previously shown that being born during the annual hungry season strongly influences susceptibility to mortality from infectious disease in young adulthood, possibly through an influence on immune function. In rural Bangladesh pregnancies are exposed to similar seasonality. The current paper uses data from a large demographic survey in the Matlab region of Bangladesh to retest the Gambian-derived hypothesis that early life exposures correlated with season of birth predict later patterns of mortality. METHODS: Since 1966, a continuous demographic surveillance system has been in operation in the rural Matlab region of Bangladesh. The current analysis is based on 172 228 births and 24 697 deaths between 1974 and 2000. Season of birth was defined as 'harvest' (January-June) and 'hungry' (July-December), based on monthly variations in rates of conception and neonatal mortality within the same dataset. RESULTS: Birth during the hungry season resulted in excess mortality during the first year of life. However, for adult mortality (deaths >15 years), there was no excess in individuals born during the annual hungry season: ratio of hazard July-December versus January-June = 1.12; 95% CI: 0.87, 1.45. CONCLUSIONS: The current study found no excess mortality in young adults born during the 'hungry' season in rural Bangladesh. This differing pattern in survival when compared with The Gambia may be a consequence of the greatly reduced incidence of young adult deaths in Bangladesh (0.1%) compared with The Gambia (3%). Under such conditions possible differences in immune function may not be detectable with early adult death as the outcome. However, it also remains possible that our Gambian observation could be a highly discrete phenomenon localized in either time or place, and as such, will not be replicated in other populations.