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Rationale: Acute respiratory infections (ARIs) are a leading cause of morbidity and mortality in children under five years of age and contribute to healthcare visits and hospitalisations globally. Vitamin D deficiency is common amongst pregnant women and young children. Given that vitamin D supplementation is safe, affordable, and easy to administer, its potential to reduce ARI-related healthcare visits holds substantial public health relevance. This review evaluates the benefit and harm of vitamin D supplementation in preventing ARI-related healthcare visits in young children, aiming to inform supplementation policies during pregnancy and early childhood. Objectives: To determine the benefit and harm of vitamin D supplementation for preventing ARIs in children up to five years of age. Search methods: We searched the Cochrane Central Register of Controlled Trials (CENTRAL), MEDLINE, Embase, CINAHL, Web of Science, WHO Global Index Medicus, and four clinical trial registries on 18 March 2025. Eligibility criteria: We included randomised controlled trials (RCTs) comparing vitamin D to placebo, or a higher dose (≥ 1000 IU) versus a lower dose (≤ 1000 IU) administered from early pregnancy through to five years of age. We excluded studies involving children with chronic respiratory or systemic conditions. Outcomes: Critical outcomes: 1) healthcare visits made by children for an ARI: (a) the proportion of children with healthcare visits made for ARIs; (b) the mean number of healthcare visits made for ARIs per child; and 2) hypercalcaemia: the number of pregnant women and children with hypercalcaemia. Risk of bias: We assessed risk of bias using the Cochrane risk of bias (RoB) 1 tool for outcomes reported in the summary of findings tables. Synthesis methods: We synthesised outcome data using meta-analyses, calculating risk ratios (RR) for dichotomous, and mean differences (MD) for continuous outcomes, each with 95% confidence intervals (CI). Where meta-analysis was not feasible, we summarised the results narratively. We used GRADE to assess the certainty of the evidence for pre-specified outcomes. Included studies: We included 107 RCTs involving 31,521 participants. Vitamin D supplementation was administered during pregnancy, early childhood, or both, and in some cases to both infants and lactating mothers. Interventions used vitamin D 2 , D 3 , both, or unspecified forms, with dosages ranging from daily administration to large single or quarterly boluses. Most were delivered orally, typically on a daily schedule, but also weekly or quarterly, over durations ranging from a single dose to 18 months. Settings included hospitals, day-care centres, communities, and homes. Synthesis of results: Vitamin D supplementation may result in a slight reduction in the proportion of children who make ARI-related healthcare visits compared to placebo (risk ratio (RR) 0.95, 95% confidence interval (CI) 0.91 to 1.00; P = 0.03; 10 studies, 2447 participants; low-certainty evidence), but probably does not reduce the mean number of ARI-related healthcare visits per child (mean difference (MD) 0.07, 95% CI -0.06 to 0.20; P = 0.32; 3 studies, 561 participants; moderate-certainty evidence). Comparisons between higher and lower doses of vitamin D probably do not reduce the proportion of children making ARI-related healthcare visits (RR 0.94, 95% CI 0.81 to 1.10; P = 0.46; 2 studies, 1382 participants; moderate-certainty evidence), and do not reduce the mean number of ARI-related healthcare visits per child (MD -0.10, 95% CI -0.59 to 0.39; P = 0.69; 1 study, 579 participants; low-certainty evidence). No cases of hypercalcaemia were reported among pregnant women in trials comparing vitamin D supplementation with placebo. In higher versus lower dose comparisons, supplementation may result in little to no difference in the risk of hypercalcaemia among pregnant women (RR 1.61, 95% CI 0.60 to 4.31; P = 0.34; 6 studies, 2379 participants; low-certainty evidence). Similarly, the evidence that vitamin D supplementation has little to no effect on the risk of hypercalcaemia in children, when compared with placebo, is very uncertain (RR 0.81, 95% CI 0.53 to 1.24; 7 studies, 1542 participants; very low-certainty evidence) and may result in little to no difference in the risk of hypercalcaemia in children when higher doses are compared to lower doses (RR 1.23, 95% CI 0.82 to 1.85; 7 studies, 1287 participants; low-certainty evidence). Authors' conclusions: We found low-certainty evidence that vitamin D supplementation during pregnancy or early childhood may result in a slight reduction in the proportion of children under five years of age who make ARI-related healthcare visits, but probably does not reduce the mean number of healthcare visits for ARIs per child. This finding highlights the need for large, well-designed, placebo-controlled trials to confirm the potential benefit. We found moderate-certainty evidence that higher-dose vitamin D supplementation, compared with lower doses, probably does not reduce the proportion of children making healthcare visits for ARIs and does not reduce the mean number of ARI-related healthcare visits per child. Hypercalcaemia occurs infrequently in both pregnant women and children receiving vitamin D supplementation. Where hypercalcaemia could be measured, specifically in children receiving vitamin D versus placebo, vitamin D supplementation may have little to no effect on the risk of hypercalcaemia (very low-certainty evidence). In both children and pregnant women, a higher dose of vitamin D compared to a lower dose may result in little to no difference in the risk of hypercalcaemia (low-certainty evidence). Funding: This Cochrane review had no dedicated funding. Registration: Protocol (2022) DOI: 10.1002/14651858.CD015111.