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Pooled review (meta-analysis)

Pooled review of 43 trials, 48,488 people: slightly fewer respiratory infections with vitamin D

Jolliffe and colleagues · The lancet. Diabetes & endocrinology · 2021

Original title: Vitamin D supplementation to prevent acute respiratory infections: a systematic review and meta-analysis of aggregate data from randomised controlled trials.

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What the study found

Points to benefit

Fewer people had one or more respiratory infections with vitamin D (61.3%) than placebo (62.3%); OR 0.92 (95% CI 0.86-0.99). Daily dosing gave OR 0.78 (0.65-0.94). Serious side effects: 0.97 (0.86-1.07).

The study at a glance

Type of study
Pooled review (meta-analysis)
Who or what was studied
43 studies, 48,488 people · People aged 0 to 95 years in 46 placebo-controlled trials
What was tested
Vitamin D3, D2 or calcifediol of any dose and length, vs placebo or low-dose vitamin D
What it measured
The question itself: colds, flu and other respiratory infections
Who has a stake
Company stake
Authors report grants from Pharma Nord, DSM Nutritional Products and Cytoplan; one author had Pfizer ties.
Journal
The lancet. Diabetes & endocrinology, 2021
Source
PubMed record 33798465. This page is based on the abstract only.

How much this study counts

  • ✓Pools the results of 43 studies with 48,488 people.
  • •A pooled review is only as strong as the studies it pools.
  • ✓Measured the question itself: colds, flu and other respiratory infections.
  • –Company stake: Authors report grants from Pharma Nord, DSM Nutritional Products and Cytoplan; one author had Pfizer ties.
  • •Summarized from the PubMed abstract only. The full paper may hold details the abstract leaves out.

✓ Strength– Limit• Note

These points come from fixed rules about the type of study, its size and length, what it measured, and who has a stake. Every study on Digestery gets the same rules.

Where this study sits

This paper is not on the ladder. It is a pooled review (meta-analysis), and reviews and opinion pieces discuss studies that are already counted on the steps below.

↑ Stronger evidence

  1. Trials in people: colds, flu and chest infections

    10

    2 benefit, 8 no difference

  2. Trials in people: other measures

    5

    1 benefit, 1 mixed, 2 no difference, 1 harm

  3. Trials in people: lab tests or safety

    1

    1 no difference

  4. Population studies

    1

    1 no difference

  5. Blood levels and genes

    4

    3 benefit, 1 no difference

↓ Weaker evidence

  • Points to benefit
  • Mixed
  • No difference
  • No direction
  • Points to harm

How it fits with the other research

The short answer to “Does vitamin D prevent colds and infections?”

Not clearly; any effect looks small.

Confidence: low. Based on 33 on-topic papers.

Read the full answer
  • Quoted in “What we found”: Earlier pools found slightly lower odds of infection: 0.88 across 25 trials in 2017, and 0.92 in 2021 (61.3% vs 62.3% of people infected).
  • Quoted in “What we found”: Vitamin D did not change how many people had serious side effects: odds of 0.96 in the 2025 pool and 0.97 in the 2021 pool.
  • Part of the finding “Pooled trials: a small benefit that faded”.
  • Part of the finding “Daily dosing and low starting levels: mixed answers”.
  • Summed up in “What reviews found”: Pooled 43 trials with 48 488 people. Slightly fewer people had an infection with vitamin D (61.3% vs 62.3%; 0.92). Daily dosing showed protection (0.78); starting vitamin D level made no difference.
  • Open item for a person: Overlap: Jolliffe 2021 updates the 2017 trials and Jolliffe 2025 adds new trials to its 43; treat the three pools as one evolving analysis.

Other reviews on this question

See all 33 studies on this question

How this page was made

Label

First read of the abstract
Claude Sonnet
Independent second read
Claude Haiku
Differences settled
The reviewer corrected 2 fields after reading the abstract. Claude Opus 5.5.
Headline
Claude Sonnet, from the checked study record. A script checked its numbers and wording.
Numbers
Checked against the abstract by a script
“How much this study counts”
Fixed rules, the same for every study
Source check by a person
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