Little empirical evidence from field-based studies exists on the relative magnitude or duration of clinical protection from Plasmodium falciparum malaria in infancy. A prospective study was undertaken to examine the age distribution of hospital admissions in four geographically and demographically well-defined areas with differing intensities of P. falciparum transmission. Where transmission was perennial, significant clinical protection from severe morbidity was observed up to the third month of life; in the seasonal transmission area, disease rates rose after the sixth month of life. Infants exposed to the highest rates of P. falciparum exposure demonstrated significant declines in the risks of severe malaria from 6 months of age. These data provide direct evidence for the very early acquisition of clinical immunity and for the existence of a period of clinical protection, which together may explain why, in these communities, the cumulative risk of malarial disease throughout childhood appears to decline with increasing transmission intensity.

Author notes

Financial support: Wellcome Trust, UK (grant 033340); Kenya Medical Research Institute; Medical Research Council, UK; and Centers for Disease Control and Prevention, National Center for Infectious Diseases, Atlanta. R.W.S., K.M., and S.G. are supported by Wellcome Trust as part of Senior Fellowships Programmes.
This paper is published with the permission of the Director of KEMRI.