Age-Patterns of Malaria Vary with Severity, Transmission Intensity and Seasonality in Sub-Saharan Africa: A Systematic Review and Pooled Analysis

Joanna Armstrong Schellenberg,Abdisalan M Noor,Brian Greenwood,Jamie T Griffin,David Schellenberg,Ilona Carneiro,Marcel Tanner,Arantxa Roca-Feltrer,Lucy Smith

doi:10.1371/journal.pone.0008988

Abstract

BackgroundThere is evidence that the age-pattern of Plasmodium falciparum malaria varies with transmission intensity. A better understanding of how this varies with the severity of outcome and across a range of transmission settings could enable locally appropriate targeting of interventions to those most at risk. We have, therefore, undertaken a pooled analysis of existing data from multiple sites to enable a comprehensive overview of the age-patterns of malaria outcomes under different epidemiological conditions in sub-Saharan Africa.Methodology/Principal FindingsA systematic review using PubMed and CAB s (1980–2005), contacts with experts and searching bibliographies identified epidemiological studies with data on the age distribution of children with P. falciparum clinical malaria, hospital admissions with malaria and malaria-diagnosed mortality. Studies were allocated to a 3×2 matrix of intensity and seasonality of malaria transmission. Maximum likelihood methods were used to fit five continuous probability distributions to the percentage of each outcome by age for each of the six transmission scenarios. The best-fitting distributions are presented graphically, together with the estimated median age for each outcome. Clinical malaria incidence was relatively evenly distributed across the first 10 years of life for all transmission scenarios. Hospital admissions with malaria were more concentrated in younger children, with this effect being even more pronounced for malaria-diagnosed deaths. For all outcomes, the burden of malaria shifted towards younger ages with increasing transmission intensity, although marked seasonality moderated this effect.ConclusionsThe most severe consequences of P. falciparum malaria were concentrated in the youngest age groups across all settings. Despite recently observed declines in malaria transmission in several countries, which will shift the burden of malaria cases towards older children, it is still appropriate to target strategies for preventing malaria mortality and severe morbidity at very young children who will continue to bear the brunt of malaria deaths in Sub-Saharan Africa.

Highlights

IntroductionThe peak prevalence of Plasmodium falciparum malaria infection is known to shift to younger age groups as transmission intensity increases [1]; there is ample evidence for this from the field [2]
The most severe consequences of P. falciparum malaria were concentrated in the youngest age groups across all settings
Despite recently observed declines in malaria transmission in several countries, which will shift the burden of malaria cases towards older children, it is still appropriate to target strategies for preventing malaria mortality and severe morbidity at very young children who will continue to bear the brunt of malaria deaths in Sub-Saharan Africa

Summary

Introduction

The peak prevalence of Plasmodium falciparum malaria infection is known to shift to younger age groups as transmission intensity increases [1]; there is ample evidence for this from the field [2]. Identification of the age groups bearing the greatest burden of clinical malaria for a given transmission setting would enable interventions to be targeted to those worst affected. It implies that reported declines in malaria transmission intensity, and future progress towards malaria elimination, will result in a shift of malaria morbidity towards older children, as has recently been observed [14,15,16].

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Conclusion