Background: A 2021 meta-analysis of 37 randomised controlled trials (RCTs) of vitamin D supplementation for prevention of acute respiratory infections (ARI) revealed a statistically significant protective effect of the intervention (odds ratio [OR] 0.92, 95% confidence interval [CI] 0.86 to 0.99). Since then, 6 eligible RCTs have completed, including one large trial (n=15,804). Methods: Updated systematic review and meta-analysis of data from RCTs of vitamin D for ARI prevention using a random effects model. Sub-group analyses were done to determine whether effects of vitamin D on risk of ARI varied according to baseline 25-hydroxyvitamin D (25[OH]D) concentration, dosing regimen or age. We searched MEDLINE, EMBASE, the Cochrane Central Register of Controlled Trials, Web of Science and the ClinicalTrials.gov were searched between May 2020 (previous search) and April 2024. No language restrictions were imposed. Double-blind RCTs supplementing vitamin D for any duration, with placebo or low-dose vitamin D control, were eligible if approved by Research Ethics Committee and if ARI incidence was collected prospectively and pre-specified as an efficacy outcome. Aggregate data, stratified by baseline 25(OH)D concentration and age, were obtained from study authors. The study was registered with PROSPERO (no. CRD42024527191). Findings: We identified 6 new RCTs (19,337 participants). Data were obtained for 16,086 (83.2%) participants in 3 new RCTs and combined with data from 48,488 participants in 43 previously identified RCTs. For the primary comparison of any vitamin D vs. placebo, the intervention did not significantly affect overall ARI risk (OR 0.94, 95% CI 0.88 to 1.00, P=0.057; 40 studies; I2 26.4%). Pre-specified subgroup analysis did not reveal evidence of effect modification by age, baseline vitamin D status, or dosing regimen. Vitamin D did not influence the proportion of participants experiencing at least one serious adverse event (OR 0.96, 95% CI 0.90 to 1.04; 38 studies; I2 0.0%). A funnel plot showed left-sided asymmetry (P=0.002, Egger's test). Interpretation: This updated meta-analysis yielded a similar point estimate for the overall effect of vitamin D supplementation on ARI risk to that obtained previously, but the 95% CI for this effect estimate now spans 1.00, indicating no statistically significant protection. Funding: None

ARM reports grants from the Fischer Family Trust, Pharma Nord Ltd, DSM Nutritional Products Ltd, the AIM Foundation, Cytoplan Ltd, and Thornton & Ross Ltd outside the submitted work. CG reports grants from the Health Technology Assessment Program of the UK National Institute of Health Research during conduct of the study. WJ reports grants from Chiesi and Astra Zeneca outside the submitted work. REN reports grants from the Australian National Health and Medical Research Council during the conduct of the study. No other author has had any financial relationship with any organisations that might have an interest in the submitted work in the previous three years. No other author has had any other relationship, or undertaken any activity, that could appear to have influenced the submitted work.

This study was conducted without external funding.

I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained.

Yes

The details of the IRB/oversight body that provided approval or exemption for the research described are given below:

Research Ethics Committee approval to conduct this meta-analysis was not required in the UK; local ethical permission to contribute data from primary trials was required and obtained for studies by Camargo et al,2 (The Ethics Review Committee of the Mongolian Ministry of Health), Murdoch et al,3 (Southern Health and Disability Ethics Committee, ref. URB/09/10/050/AM02), Rees et al,4 (Committee for the Protection of Human Subjects, Dartmouth College, USA; Protocol # 24381), Tachimoto et al,5 (Ethics committee of the Jikei University School of Medicine, ref 26-333: 7839), Tran et al,6 (QIMR Berghofer Medical Research Institute Human Research Ethics Committee, P1570) and Urashima et al,7,8 (Ethics committee of the Jikei University School of Medicine, ref 26-333: 7839).

I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals.

Yes

I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance).

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I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable.

Yes

the study dataset is available from d.a.jolliffe@qmul.ac.uk.