Background
Encouraging progress has been seen with reductions in Plasmodium falciparum malaria
transmission in some parts of Africa. Reduced transmission might lead to increasing susceptibility
to malaria among older children due to lower acquired immunity, and this has
implications for ongoing control strategies.
Methods and Findings
We conducted a longitudinal observational study of children admitted to Kilifi County Hospital
in Kenya and linked it to data on residence and insecticide-treated net (ITN) use. This
included data from 69,104 children aged from 3 mo to 13 y admitted to Kilifi County Hospital
between 1 January 1990 and 31 December 2014. The variation in malaria slide positivity
among admissions was examined in logistic regression models using the following predictors:
location of the residence, calendar time, the child’s age, ITN use, and the enhanced
vegetation index (a proxy for soil moisture). The proportion of malaria slide-positive admissions
declined from 0.56 (95% confidence interval [CI] 0.54–0.58) in 1998 to 0.07 (95% CI
0.06–0.08) in 2009 but then increased again through to 0.24 (95% CI 0.22–0.25) in 2014.
Older children accounted for most of the increase after 2009 (0.035 [95% CI 0.030–0.040]
among young children compared to 0.22 [95% CI 0.21–0.23] in older children). There was a
nonlinear relationship between malaria risk and prevalence of ITN use within a 2 km radius
of an admitted child’s residence such that the predicted malaria positive fraction varied from
~0.4 to <0.1 as the prevalence of ITN use varied from 20% to 80%. In this observational analysis, we were unable to determine the cause of the decline in malaria between 1998
and 2009, which pre-dated the dramatic scale-up in ITN distribution and use.
Conclusion
Following a period of reduced transmission, a cohort of older children emerged who have
increased susceptibility to malaria. Further reductions in malaria transmission are needed
to mitigate the increasing burden among older children, and universal ITN coverage is a
promising strategy to achieve this goal.