LONDON - Boosting levels of vitamin D didn't help prevent Covid-19 in a pair of large studies conducted in the UK and Norway, highlighting the importance of immunisation.
Supplementing normal vitamin D intake with pills or cod liver oil, a source of vitamin D, had no impact on whether individuals would test positive for Covid-19 or other acute respiratory tract infections, according to the studies published Thursday in the BMJ medical journal.
Covid-19 shots remain most effective for protecting people from the coronavirus, and doctors shouldn't offer vitamin D supplements to healthy people with normal levels, Dr Peter Bergman, a professor at the Karolinska Institute in Sweden, said in an accompanying editorial in the same journal.
The vitamins should be given to risk groups including people with dark skin, those who normally don't get out into the sun, pregnant women and some elderly people with chronic diseases, Dr Bergman wrote.
Researchers began studying vitamin D and Covid-19 after some reports that people with low levels were at elevated risk of infection and more severe symptoms of the disease.
Deficiency in the vitamin is more common among Black and Hispanic Americans than their White counterparts who have been at lower risk of contracting Covid-19, further prompting inquiry about whether supplements can ward off the disease.
Conducted from December 2020 to June 2021, the UK trial included some 6,200 people age 16 and older who hadn't been using vitamin D supplements beforehand. Almost half of the participants received vitamin D supplements for six months.
There was no effect on diagnosed acute respiratory tract infections or Covid-19 cases - tested by either lateral flow assay or more accurate PCR - during a six-month follow-up period.
The Norway trial ran from November 2020 to June 2021 and included about 35,000 people between the ages of 18 to 75.
Over six months, participants received either 5mm of cod liver oil - which contains low doses of vitamin D - every day or 5mm of corn oil as a placebo.
The researchers found no effect of cod liver oil on acute respiratory infections or confirmed Covid cases compared to the placebo group.
Among the trials' limitations were that both took place as countries rolled out vaccines against Covid-19.
In the UK trial, people knew whether or not they were taking vitamin D, and almost half of people in the control group acknowledged taking a vitamin D supplement at least once during the study.
Participants in the Norwegian study were mostly young, healthy and started off with adequate levels of vitamin D. BLOOMBERG