Analysis of immunity to febrile malaria in children that distinguishes immunity from lack of exposure

Abstract

In studies of immunity to malaria, the absence of febrile malaria is commonly considered evidence of "protection." However, apparent "protection" may be due to a lack of exposure to infective mosquito bites or due to immunity. We studied a cohort that was given curative antimalarials before monitoring began and documented newly acquired asymptomatic parasitemia and febrile malaria episodes during 3 months of surveillance. With increasing age, there was a shift away from febrile malaria to acquiring asymptomatic parasitemia, with no change in the overall incidence of infection. Antibodies to the infected red cell surface were associated with acquiring asymptomatic infection rather than febrile malaria or remaining uninfected. Bed net use was associated with remaining uninfected rather than acquiring asymptomatic infection or febrile malaria. These observations suggest that most uninfected children were unexposed rather than "immune." Had they been immune, we would have expected the proportion of uninfected children to rise with age and that the uninfected children would have been distinguished from children with febrile malaria by the protective antibody response. We show that removing the less exposed children from conventional analyses clarifies the effects of immunity, transmission intensity, bed nets, and age. Observational studies and vaccine trials will have increased power if they differentiate between unexposed and immune children.

FIG. 1.

Proportions of children having asymptomatic or febrile malaria and of children remaining malaria free over 3 months of observation sorted by age group. Age groups are defined according to quartiles. An asterisk indicates statistical significance (P < 0.05) for logistic regression for a shift in the balance between asymptomatic parasitemia and febrile malaria or between asymptomatic parasitemia and uninfected status (P < 0.05).

FIG. 2.

Proportions of children having asymptomatic or febrile malaria and of children remaining malaria free over 3 months of observation sorted by anti-VSA antibody level, bed net use, and parasitemia. Tertiles of the antibody response to VSA are used, along with bed net use and parasitemia at the first cross-sectional survey (before curative treatment was given). An asterisk indicates statistical significance (P < 0.05) for logistic regression for a shift in the balance between asymptomatic parasitemia and febrile malaria or between asymptomatic parasitemia and uninfected status (P < 0.05).

FIG. 3.

Proportions of children having asymptomatic or febrile malaria and of children remaining malaria free over 3 months of observation sorted by village. An asterisk indicates statistical significance (P < 0.05) for logistic regression for a shift in the balance between asymptomatic parasitemia and febrile malaria or between asymptomatic parasitemia and uninfected status (P < 0.05).

FIG. 4.

Survival plots for time to febrile malaria before and after “unexposed” children were excluded. Statistically significant differences (P < 0.05) as determined by Cox regression are indicated by an asterisk. Children are divided into two categories according to age (old [44 to 82 months] versus young [12 to 44 months]), according to levels of antibody to VSA (high [15 to 85% recognition] versus low [0 to 15% recognition]), according to village (high transmission versus low transmission), and according to bed net use. The plots are survival plots for each categorization of children. Each plot shows the results of the analysis of all subjects and also a repeated analysis after the unexposed children were excluded.