This is a two-arm, cluster-randomised, phase IV trial conducted in Chad to assess the protective efficacy and impact in real-life conditions of a new strategy for administering the R21/MM malaria vaccine, synchronized within a seasonal malaria chemoprevention (SMC) campaign, among children living in areas of high seasonal malaria transmission. In this study, a cluster is defined as the catchment area of a primary care health centre. In Chad, each catchment area is known as a 'zone of responsibility' (French: Zone de Responsibilité' \[ZR\]). Twenty-six (26) of the total 27 ZRs in the districts of Moïssala and Dembo will be randomized in a 1:1 ratio to receive a 4-dose (3 primary doses + 1 booster) R21/MM schedule either (1) integrated into the routine EPI vaccination program (the Routine control arm), or (2) synchronized with an annual seasonal malaria chemoprevention (SMC) campaign (the Synchronized intervention arm). Malaria incidence: R21/MM effectiveness will be assessed using the incidence of biologically confirmed clinical malaria (trial primary endpoint). The incidence of clinical malaria will be determined through enhanced surveillance of malaria cases in health centres and hospitals over a 17-month period (August 2025 - December 2026). Coverage surveys: Cross-sectional surveys (cluster sampling) will be carried out to measure R21/MM vaccine coverage, SMC coverage, coverage of other malaria prevention measures, and coverage of other EPI vaccines. Nested case-control study: A sub-sample of children admitted to Moïssala District Hospital with severe clinical malaria will be offered the opportunity to participate in a nested case-control study designed to estimate the individual protective efficacy of R21/MM against severe malaria.