Age-dependent susceptibility to severe disease with primary exposure to Plasmodium falciparum.

This study investigated the incidence of severe disease following primary exposure to Plasmodium falciparum by nonimmune children and adults in Irian Jaya, Indonesia. Four months after arrival, the cross-sectional prevalence of P. falciparum was 72%, and the monthly cumulative incidence of clinical diagnoses of malaria was 81%. Delirium or unconsciousness prompted evacuation to the hospital. Records of emergency evacuation of persons with a clinical diagnosis of malaria revealed an incidence density among adults (>15 years) of 1.34 events/person-year in the third month, whereas the rate in children remained stable at approximately 0.25 events/person-year (relative risk = 4.51, 95% confidence interval [CI] = 1.94-11). Through the first 6 months of exposure, 23.2% of adults were evacuated to the hospital with a diagnosis of malaria compared with 8.6% of children (relative risk = 2.7, 95% CI = 1.9-3.8). In this population with relatively few infants or people of advanced age, the risk of severe disease following primary exposure to P. falciparum increased with age.