In this cohort, α+-thalassaemia was associated with increased rates of malaria in children aged < 18 months, but with protection against malaria in older children. There was no evidence that α+-thalassaemia was associated with the severity of malaria episodes as measured by haemoglobin concentrations and other indicators.
Although all Plasmodium-infected children were treated at baseline, the specificity of our case definition may have gradually decreased with time, as more children became asymptomatically infected. Similar results were obtained, however, when restricting the analysis to cases with parasite densities > 10,000/μL or > 100,000/μL. Because these case definitions are more specific for detecting true malaria cases, it is unlikely that a low specificity affected the validity of the conclusions drawn in this report.
Because effect estimates were adjusted for factors associated with malaria risk (including distance to the research facility and intervention), it is unlikely that a difference in external factors (such as intervention, exposure to infection or health seeking behaviour) biased the estimates and would be responsible for the increased risk associated with α+-thalassaemia in the youngest children. It remains unclear however, what mechanism could underlie the increased incidence in the youngest children. It has been put forward that Plasmodium parasites preferentially invade reticulocytes [26, 27]. Thus reticulocytosis, induced by thalassaemia-associated ineffective erythropoiesis, would favour proliferation of Plasmodium parasites . However, there was no strong support that thalassaemia was associated with increased parasite densities in young children, and neither did any previous study. In addition, a recent study found no evidence that reticulocyte counts were increased in individuals with α+-thalassaemia .
Others hypothesized that increased parasite-induced surface expression of neo-antigens on thalassaemic erythrocytes results in enhanced binding of IgG antibody and more rapid clearance of parasitized erythrocytes in the spleen . Clearance in the spleen may be further enhanced by a reduced erythrocyte deformability of thalassaemic cells . In children aged 6-18 months, in whom protection by maternal antibodies has waned, but acquired immunity is still low and parasite replication only partly restrained by an effective circulating antibody repertoire, such increased antigen presentation in the spleen may result in a more rapid development of symptoms, and at the same time a more efficient and more rapid acquisition of protective immunity. Further evidence to support this theory is, however, lacking.
Nevertheless, the findings reported here contribute to existing epidemiological evidence that predisposition to malaria due to P. falciparum in both heterozygotes and homozygotes for α+-thalassaemia early in life may result in protection against severe malaria, and (at older age) uncomplicated malaria due to the same species. In Vanuatu, the incidence of malaria due to P. falciparum and P. vivax was increased in children aged < 5 years with homozygous α+-thalassaemia relative to children with normal genotype , but the study found no evidence of protection among either hetero- or homozygotes in children aged 5-9 years. Contrary to our findings, the incidence in heterozygotes and children with normal genotype were similar, regardless of whether the analysis was stratified by age or not. The estimates in the current report are more precise, however, due to the larger number of malaria cases in this study (622 and 812 in children with heterozygote and normal genotypes, respectively, versus 159 and 304 in Vanuatu). In an area adjacent that used in the current report, with similar levels of malaria endemicity, α+-thalassaemia was found to be associated with protection against malaria in children aged 6 months to 20 years . This protection seemed more pronounced among children aged > 5 years, but the analysis was based on 50 episodes (41 among children aged < 5 years) and had insufficient precision to adequately assess age-specific effects in early life.
Because the intensity of malaria exposure determines how fast protective immunity is obtained, the age at which a difference in protective immunity between children with and without α+-thalassaemia becomes evident shall vary with transmission intensity. This may at least in part explain differences between studies; under conditions of intense transmission, such as in the present study, the difference attained would become evident earlier in life than in conditions of less intense transmission such as encountered in Vanuatu .
The finding that α+-thalassaemia is associated with an increased frequency of malaria in children aged 6-18 months may seem to contradict reports from hospital-based studies that α+-thalassaemia protects against severe malarial anaemia [4, 6], which has the highest incidence in the same age range . An increased fever rate may, however, not necessarily translate to an increased risk of severe malaria anaemia if the decline in haemoglobin during these attacks is halted or sufficiently slowed down to before reaching a critical level that leads to admission. A potential mechanism for such phenomenon has recently been proposed . In thalassaemia, the total amount of haemoglobin is divided over erythrocytes that are disproportionately increased in numbers but reduced in size and haemoglobin content. Thus, an equal proportion of erythrocytes being destroyed by malaria parasites results in a smaller haemoglobin reduction in individuals with α+-thalassaemia than in their peers with normal genotype.
When analysing the decline in haemoglobin concentration between baseline and first malaria episode, we did not find evidence of such protection. It should be noted, however, that the average decline in haemoglobin concentration during malaria episodes was relatively small, and that very few (19) episodes occurred whereby haemoglobin concentrations dropped below 60 g/L. This is probably due to good access to treatment in our study, and haematological gains due to the nutrient interventions. By contrast, in a pilot survey in the same area in 2006, children with α+-thalassaemia were found to be protected against the decline in haemoglobin concentration associated with mild and asymptomatic infections . At that time, there were no first-line health facilities in the area, and artemether-lumefantrine was not available through public facilities. Thus this setting was probably better comparable to the circumstances under which α+-thalassaemia has been providing a survival advantage in the past.
A likely explanation for the increased rates of episodes with haemoglobin concentrations < 80 g/L observed in heterozygotes and homozygotes (Table 2) is that children with these genotypes had lower initial haemoglobin concentrations before the onset of malaria episodes than children with normal genotype. Thus a reduction in haemoglobin concentration caused by malaria in heterozygotes and homozygotes will more readily result in haemoglobin concentrations dropping below a threshold of 80 g/L.