title: Evidence for over-dispersion in the distribution of clinical malaria episodes in children.
abstract: BACKGROUND
  It may be assumed that patterns of clinical malaria in children of similar age under the same level of exposure would follow a Poisson distribution with no over-dispersion. Longitudinal studies that have been conducted over many years suggest that some children may experience more episodes of clinical malaria than would be expected. The aim of this study was to identify this group of children and investigate possible causes for this increased susceptibility.
  
  
  RESULTS
  Using Poisson regression, we chose a group of children whom we designated as 'more susceptible' to malaria from 373 children under 10 years of age who were followed up for between 3 to 5 years from 1998-2003. About 21% of the children were categorized as 'more susceptible' and although they contributed only 23% of the person-time of follow-up, they experienced 55% of total clinical malaria episodes. Children that were parasite negative at all cross-sectional survey were less likely to belong to this group [AOR = 0.09, (95% CI: 0.14-0.61), p = 0.001].
  
  
  CONCLUSIONS
  The pattern of clinical malaria episodes follows a negative binomial distribution. Use of lack of a clinical malaria episode in a certain time period as endpoints for intervention or immunological studies may not adequately distinguish groups who are more or less immune. It may be useful in such studies, in addition to the usual endpoint of the time to first episode, to include end points which take into account the total number of clinical episodes experienced per child.
authors.affiliation: Kenya Medical Research Institute, Centre for Geographic Medicine Research, Coast/Wellcome Trust Collaborative Program, Kilifi, Kenya. tmwangi@kilifi.kemri-wellcome.org
authors.forename: Tabitha Wanja
authors.initials: TW
authors.lastname: Mwangi
authors.forename: Gregory
authors.initials: G
authors.lastname: Fegan
authors.forename: Thomas Neil
authors.initials: TN
authors.lastname: Williams
authors.forename: Sam Muchina
authors.initials: SM
authors.lastname: Kinyanjui
authors.forename: Robert William
authors.initials: RW
authors.lastname: Snow
authors.forename: Kevin
authors.initials: K
authors.lastname: Marsh
mesh_terms: D002648:Child; D004198:Disease Susceptibility; D006801:Humans; D008288:Malaria / Q000453:epidemiology*; D016012:Poisson Distribution; D012307:Risk Factors