Resistance of Plasmodium falciparum to sulfadoxine–pyrimethamine threatens the antimalarial effectiveness of intermittent preventive treatment during pregnancy (IPTp) with sulfadoxine–pyrimethamine (ITPp-SP) in sub-Saharan Africa. We updated an aggregated-data meta-analysis to assess the associations between sulfadoxine–pyrimethamine resistance and the effectiveness of IPTp-SP to inform policy.

We searched databases (Jan 1, 1990, to June 8, 2024) for observational studies or trials reporting data on malaria, low birthweight (<2500 g), anaemia, and other outcomes by IPTp-SP dose and matched these by year and location with studies that reported on molecular markers of sulfadoxine–pyrimethamine resistance. Studies including only women with HIV or combined interventions were excluded. We evaluated how sulfadoxine–pyrimethamine resistance influenced the adjusted risk ratio (aRR) between three and two doses of IPTp-SP for various outcomes using Poisson mixed-effects models that allowed for non-linear relationships. Initially, we performed a threshold analysis, stratified by region, to identify the resistance levels most predictive of altered effect of IPTp-SP doses on malaria parasitaemia at delivery (peripheral or placental parasitaemia by any test), our primary outcome. These resistance strata were then used in all subsequent models for other outcomes. All analyses were adjusted for malaria transmission intensity, HIV infection, percentage of paucigravidae, and insecticide-treated net use. Performance of models was evaluated using cross-validation. The trial was registered with PROSPERO (CRD42021250359).

Overall, 122 studies involving 148 693 participants were included. For west and central Africa (69 studies comprising 63 745 participants), very low resistance was categorised as a prevalence of the dihydropteroate synthase (dhps) Lys540Glu mutation in the parasite population of less than 4%, and low resistance as a prevalence of Lys540Glu of 4% or higher. In east and southern Africa (53 studies comprising 84 948 participants), moderate resistance was categorised as a prevalence of the Lys540Glu mutation of less than 60% combined with a prevalence of the Ala581Gly mutation of less than 5%, high resistance as a prevalence of Lys540Glu of 60% or higher combined with a prevalence of Ala581Gly of less than 5%, and very high resistance as a prevalence of the Lys540Glu mutation of 60% or higher combined with a prevalence of Ala581Gly of 5% or higher. There was a marked trend towards lower efficacy of IPTp-SP on reducing malaria infection with increasing resistance levels. In west and central Africa, when comparing three versus two doses, the aRR was 0·71 (95% CI 0·65–0·78) in areas with very low resistance and 0·83 (0·72–0·95) in areas with low resistance (p=0·0144 for the difference between dose–response curves in very low vs low resistance). For east and southern Africa, the same trend was observed: the aRR was 0·63 (95% CI 0·57–0·69) in areas with moderate resistance, 0·89 (0·82–0·96) in areas with high resistance, and 0·93 (0·85–1·01) in areas with very high resistance (p<0·0001 for dose–response curves differences between moderate vs high and moderate vs very high resistance). This pattern was not seen for low birthweight. When comparing three versus two doses in west and central Africa, the aRR was 0·58 (95% CI 0·48–0·68) in areas with very low resistance and 0·56 (0·44–0·68) in areas with low resistance (p=0·72 for dose–response curves very low vs low resistance). For east and southern Africa, the aRR was 0·75 (95% CI 0·52–0·98) in areas with moderate resistance, 0·73 (0·69–0·78) in areas with high resistance, and 0·75 (0·63–0·87) in areas with very high resistance (p=0·80 for dose–response curves moderate vs high resistance; p=0·90 for moderate vs very high resistance). Dose comparisons in some resistance strata were limited by sample size.

IPTp-SP antimalarial efficacy is greatly reduced in very high resistance areas. However, it remains effective at reducing low birthweight in these areas, possibly through non-malaria effects on fetal growth. While IPTp-SP use should continue in high SP-resistance areas, alternative malaria preventive strategies are urgently needed in these areas.

WHO and WorldWide-Antimalarial-Resistance-Network.