Respiratory · VITD-RESP-01 · v2
This corpus contains many randomised trials that gave people vitamin D to see whether they caught fewer respiratory infections. The biggest and most carefully run trials found no reduction in infection risk [15][16][17][5]. Smaller trials, and analyses limited to people who started out low in vitamin D, did sometimes show fewer infections [1][8][18], and one trial in children found fewer non-influenza viruses while influenza was unchanged [10]. One trial even recorded more upper respiratory infections in the supplemented group [6]. Because the claim is stated for everyone without conditions, and the strongest trials do not support a general benefit, the panel treated it as not holding as written and saw any real effect as likely confined to people who were deficient.
The corpus is dominated by direct vitamin D ARI-prevention RCTs, and they split by size and population. The four largest trials show no reduction: cod liver oil in 34,601 adults [15], D-Health in 21,315 older adults (OR 0.98) [16], Camargo in 5,110 adults (HR 1.01) [17], and CORONAVIT in 6,200 adults [5]. Benefit appears in smaller or selected settings: long-term-care residents (IRR 0.60) [1], IBD patients (RR 0.59, stronger at low 25-OHD, RR 0.36) [8], and military trainees (URTI days reduced 36%) [18]. In children, influenza was unchanged while all-virus infection fell modestly (HR 0.81) [10], and ViDiFlu recorded increased URI risk (aHR 1.48) [6]. The contested point: The division is between trial size and baseline vitamin D status: the large, largely replete-population trials are null [15][16][17][5], while positive signals cluster in smaller trials and in deficient subgroups [1][8][18]. The dispute is therefore whether any genuine effect exists at all for the general population, or only as a conditional benefit in deficiency.
All 21 papers behind this claim with their DOIs and what each does to it, the panel’s arbitration, the evidence gap map and the open research questions. Free, and sent to your inbox.
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One reviewer's only job is to attack the finding. The challenges below were weighed before the grade was set.
The claim is unconditional, but the four largest and best-powered trials ([15] n=34,601; [16] n=21,315; [5] n=6,200; [17] n=5,110) all show no reduction in ARI risk. Weight of evidence opposes a general effect.
Positive effects cluster in vitamin-D-deficient subgroups ([8] RR 0.36 only in low 25-OHD; [17] still null even in its deficient subgroup). The blanket claim omits this essential precondition.
Where it remains uncertain. Positive results concentrate in small trials and low-25-OHD subgroups ([8] RR 0.36 at low 25-OHD), and one trial recorded increased URI risk [6], so the unconditional claim is broader than the evidence supports.
Aleatory
Positive effects come from small trials ([1] n=107, [12] n=86) where random variation can produce apparent benefit.
Epistemic
Whether any benefit exists outside vitamin-D-deficient individuals is unresolved; large trials enrolled largely replete populations [16][17].
Inferential
Lab-confirmed swabs [2][10], parent/self-report [15][16], primary-care visits [9] and symptom days [18] are pooled as one 'ARI' outcome though they measure different things.
The load-bearing papers from a corpus of 21, each classified by what it does to the claim. Every DOI is taken from the corpus the claim was graded on.
Positive: high-dose vitamin D reduced ARI incidence in elderly long-term care residents, but n=107.
“The high-dose group had 0.67 ARIs per person-year and the standard-dose group had 1.11 (incidence rate ratio (IRR) = 0.60, 95% confidence interval (CI) = 0.38-0.94, P = .02).”
Panel auditInternal validity: medium · scope of inference: Long-term care residents aged 60 and older in Colorado facilities over 12 months.Key limitations: small n · high-dose vs standard-dose not vs no supplement · higher fall rate in high-dose arm
Null in a pragmatic population test-and-treat design (n=6,200).
“implementation of a population level test-and-treat approach to vitamin D supplementation was not associated with a reduction in risk of all cause acute respiratory tract infection or covid-19.”
Panel auditInternal validity: high · scope of inference: UK adults aged 16+ with high baseline prevalence of suboptimal vitamin D status, over six months.Key limitations: open-label · test-and-treat offer not per-protocol supplementation · point estimates slightly above 1
Null for ARI and associated with increased URI risk — evidence against benefit.
“The active intervention did not influence time to first ARI (adjusted HR (aHR) 1.18, 95% CI 0.80 to 1.74, p=0.42).”
Panel auditInternal validity: medium · scope of inference: Older adults in sheltered accommodation and their carers in London over one year.Key limitations: bolus added to daily dose, not supplement vs none · active arm had increased URI risk and duration
Null in young children (incidence RR 0.97).
“daily administration of 2000 IU compared with 400 IU of vitamin D supplementation did not reduce overall wintertime upper respiratory tract infections.”
Panel auditInternal validity: high · scope of inference: Healthy children aged 1 to 5 years over wintertime in Toronto.Key limitations: high-dose vs standard-dose, both arms supplemented · both arms achieved sufficient 25(OH)D
4 of 21 papers shown. The full review lists all of them, with the panel’s arbitration →
Claims are graded at each quarterly release, and re-graded between releases only when a pivotal trial lands. Every change is a new version of the claim.
The best-powered RCTs show no overall ARI risk reduction and benefit is conditional on baseline deficiency, so the blanket claim leans toward not being true on this corpus.
Release 2026-q4 re-graded every claim from its corpus under protocol v0.19. An earlier release's grade is shown for the record; it came from an earlier protocol and is not a movement.
Assumptions that carry weight but are not themselves established.
The panel records the gap that would most move the claim, and the study that would close it.
Deficiency-restricted prevention trial. Benefit signals cluster at low baseline 25-OHD [8], but the large null trials enrolled largely replete populations [16][17], so the deficiency hypothesis is untested at scale. The panel rates this the highest-importance open gap for the claim, and names the study design that would close it: A large RCT enrolling only participants with confirmed low 25-OHD, with a prespecified lab-confirmed ARI incidence endpoint.
Open question: Does vitamin D supplementation reduce ARI risk when restricted to confirmed vitamin-D-deficient individuals? Why it is still open: Positive effects appear in low-25-OHD subgroups [8], but the large trials that were null enrolled largely replete populations [16][17] and [17] remained null even in its deficient subgroup.
Priors is a sceptical first reader, not peer review, and not medical advice.
Release 2026-q4 · graded 26 Oct 2026 · protocol v0.19