Household-level effects of seasonal malaria chemoprevention in the Gambia

Abstract

Background: In 2022 the WHO recommended the discretionary expansion of the eligible age range for seasonal malaria chemoprevention (SMC) to children older than 4 years. Older children are at lower risk of clinical disease and severe malaria so there has been uncertainty about the cost-benefit for national control programmes. However, emerging evidence from laboratory studies suggests protecting school-age children reduces the infectious reservoir for malaria and may significantly impact on transmission. This study aimed to assess whether these effects were detectable in the context of a routinely delivered SMC programme.

Methods: In 2021 the Gambia extended the maximum eligible age for SMC from 4 to 9 years. We conducted a prospective population cohort study over the 2021 malaria transmission season covering 2210 inhabitants of 10 communities in the Upper River Region, and used a household-level mixed modelling approach to quantify impacts of SMC on malaria transmission.

Results: We demonstrate that the hazard of clinical malaria in older participants aged 10+ years ineligible for SMC decreases by 20% for each additional SMC round per child 0-9 years in the same household. Older inhabitants also benefit from reduced risk of asymptomatic infections in high SMC coverage households. Spatial autoregression tests show impacts are highly localised, with no detectable spillover from nearby households.

Conclusions: Evidence for the transmission-reducing effects of extended-age SMC from routine programmes implemented at scale has been previously limited. Here we demonstrate benefits to the entire household, indicating such programmes may be more cost-effective than previously estimated.

Fig. 1. Map of study site.

The locations of the ten study communities are denoted by black markers in the Upper River Region of The Gambia. The inset shows the location of the region within the country.

Fig. 2. Flow diagram of study participants and households.

All residents of the study site were eligible to participate in the study and 87% were enroled. Only participants who were present in at least one survey during the 2021 malaria season (July-December) with full address data were included in the final analysis sample.

Fig. 3. Incidence of clinical malaria by household-level coverage of SMC: Kaplan-Meier survival plots of clinical malaria episodes in 1385 study participants aged 10+ years by SMC coverage in eligible children aged 0–9 years in the same household.

Vertical axes start at ‘60% no clinical malaria at time = t’ for clarity. SMC coverage is defined as (a) percentage of eligible children in household with 1+ rounds of SMC, where the blue line is participants in households with <25% coverage, the orange line refers to those in households with 25–<80% coverage and the green line those with 80%+ coverage. b Mean number of SMC rounds per eligible child, were blue line is participants in households with <0.3 rounds/child, the orange line refers to those in households with 0.3–<2 rounds per child, and the green line those with 2+ rounds per child. Grey vertical lines indicate the first reported day of each monthly round of SMC.

Fig. 4. Prevalence and density of asymptomatic infections in 1,056 study participants aged 10+ years surveyed between the 27th Sept and 27th Nov 2021, by household-level coverage of SMC in young children.

a Prevalence of asymptomatic infection in all participants aged 10+ years, by the coverage of SMC in young children in the same household. SMC coverage is defined as the percentage of children 0–9 years who received at least one round of SMC in three groups: <25% (blue bar), 25–75% (orange bar) and 80%+ (green bar). b Kernel density plot showing the log (natural) parasite concentrations originally measured in counts per μL in PCR-positive participants aged 10+ years. Plots are stratified by SMC coverage in children 0–9 years in the same household, defined as the percentage of children 0–9 years who received at least one round of SMC in three groups: <25% (blue plot), 25–75% (orange plot) and 80%+ (green plot). Median parasite concentrations for each SMC coverage group are shown by dotted vertical lines (colours to match SMC coverage category); cut-offs for high-density infections (averaged for age groups 10–15 and 16+ years) are shown as solid black lines. c Prevalence of asymptomatic infection in all participants aged 10+ years, by the coverage of SMC in young children in the same household. SMC coverage is defined as the mean number of SMC rounds per child also in three groups: <0.3 (blue bar), 0.3–2 (orange bar) and >2 (green bar). d Kernel density plot showing the log (natural) parasite concentrations originally measured in counts per μL in PCR-positive participants aged 10+ years. Plots are stratified by SMC coverage in children 0–9 years in the same household, defined as the mean number of SMC rounds per child also in three groups: <0.3 (blue plot), 0.3–2 (orange plot) and >2 (green plot). Median parasite concentrations for each SMC coverage group are shown by dotted vertical lines (colours to match SMC coverage category); cut-offs for high-density infections (averaged for age groups 10–15 and 16+ years) are shown as solid black lines.