Findings from two large clinical trials published yesterday in BMJ discount the notion that vitamin D supplements protect against COVID-19 or other respiratory-tract infections, although both had notable limitations.

No differences in vitamin, placebo results

In a phase 3 randomized, controlled trial in the United Kingdom, a team led by University of London researchers enrolled 6,200 participants aged 16 years and older, tested the vitamin D levels of 3,100, and gave a 6-month supply of two different doses of oral supplements to 2,674 with suboptimal concentrations (less than 75 nanograms per deciliter [ng/dL]). The control group consisted of 2,949 participants not offered vitamin D.

Median participant age was 60.2 years, 67.0% were female, and 1.2% had received at least one dose of a COVID-19 vaccine at enrollment, a figure that rose to 89.1% by study end. Supplement recipients knew they were taking an active drug, and nearly half of controls reported taking a vitamin D supplement at least once during the study.

Participants, who reported taking no vitamin D supplements at enrollment, were given either 800 or 3,200 international units [IU]/day for 6 months. Follow-up ran from Dec 17, 2020, to Jun 16, 2021, a period of high COVID-19 prevalence and low COVID-19 vaccine coverage in the United Kingdom. During the 6-month follow-up, participants received monthly online questionnaires about any respiratory-tract or COVID-19 infections and symptoms.

A total of 299 participants tested positive for an all-cause respiratory infection. Relative to controls, at least one acute respiratory infection was diagnosed in 5.7% of the lower-dose group (odds ratio [OR], 1.26; 95% confidence interval [CI], 0.96 to 1.66) and in 5.0% of the higher-dose group (OR, 1.09; 95% CI, 0.82 to 1.46).

Compared with the 78 of 2,949 infected controls (2.6%), 55/1,515 (3.6%) in the lower-dose group tested positive for COVID-19 (OR, 1.39; 95% CI, 0.98 to 1.97), as did 45/1,515 (3.0%) in the higher-dose group (OR, 1.13; 95% CI, 0.78 to 1.63).

A subset of participants underwent vitamin D testing at the end of the study. Relative to controls, vitamin D levels were, on average, 12.7 ng/dL higher in the lower-dose group and 36.3 ng/dL higher in higher-dose recipients. A subgroup analysis found no evidence that COVID-19 vaccination changed the effect of allocation or COVID-19 incidence.

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