In two studies from the UK and Norway, scientists found that boosting vitamin D levels in adults during the SARS-CoV-2 pandemic was not linked to protection against respiratory tract infections or COVID-19.
Vitamin D metabolites have long been recognized to support innate immune responses to respiratory viruses and bacteria.
Some studies have shown that vitamin D may protect against respiratory tract infections, particularly among those who are vitamin D-deficient.
Vitamin D has therefore received much attention for its potential role in preventing and treating COVID-19, but most studies published so far are observational and have shown mixed results.
In the first study, the team examined 6,200 adults (16 years and over) who did not using vitamin D supplements at enrolment.
Half (3,100 participants) were offered a vitamin D blood test and those found to have low vitamin D levels (2,674; 86%) received either 3200 IU/day or 800 IU/day of vitamin D supplements for six months, while the other half (controls) received no test or supplements.
The team found neither of the vitamin D doses showed any effect on diagnosed acute respiratory tract infections or lateral flow test or RT-PCR-confirmed COVID-19 cases over a six-month follow-up period.
The number of adverse events was similar between groups, and no serious adverse event was attributed to study supplements.
The second study was conducted in Norway between November 2020 and June 2021 using cod liver oil, which contains low doses of vitamin D and vitamin A as well as omega-3 fatty acids.
A total of 34,741 adults (18-75 years) who were not using vitamin D supplements received either 5 mL cod liver oil or 5 mL placebo (corn oil) daily for six months.
The majority of participants (86%) who were tested had adequate vitamin D levels at the start of the study.
Again, the researchers found no effect of cod liver oil on acute respiratory infections or PCR-confirmed COVID-19, compared with a placebo.
The cod liver oil group had no more side effects than the placebo group and only low-grade side effects were reported.
The results are in line with previous research that found no preventive effect of vitamin D on risk of COVID-19.
Together, these new results suggest that vitamin D supplements do not reduce risk of COVID-19 or other acute respiratory infections.
The researchers say the findings should also be interpreted in the context of a highly effective vaccine rolled out during both trials.
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The study was published in The BMJ.
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