A global collaboration that looked inside the data
For years, doctors noticed that people with low vitamin D levels seemed to get more colds, flu, and other acute respiratory infections. But the evidence was messy. Some clinical trials showed protection from vitamin D supplements, others did not.
To clear this up, a global research team led by Adrian Martineau of Queen Mary University of London and Carlos Camargo of Massachusetts General Hospital pulled together raw data from 25 randomized controlled trials. These trials included more than 11,000 people from over a dozen countries, including the United States, Canada, and the United Kingdom.
Instead of just comparing averages from each trial, the team used an individual participant data meta analysis. That means they analyzed the results for each person across all the studies. This approach gives a higher resolution picture and can reveal patterns that are hidden in trial level summaries.
Their conclusion, published in The BMJ in 2017, was clear. Vitamin D supplementation helped protect against acute respiratory infections. Camargo put it simply: most people know vitamin D is critical for bone and muscle health, and now the analysis showed that it also helps the body fight infections that cause millions of deaths worldwide every year.
What the 2017 analysis actually found
The team examined randomized, double blind, placebo controlled trials that used vitamin D2 or D3 for people from infancy to old age. In total, data from 10,933 participants were included, covering 25 trials.
Across all participants, vitamin D supplements reduced the risk of having at least one acute respiratory infection. The adjusted odds ratio was 0.88, with a 95 percent confidence interval from 0.81 to 0.96. That means people taking vitamin D had about a 12 percent lower odds of getting an infection compared to those taking placebo, and the result was statistically significant.
The pattern became even stronger when the researchers looked at how vitamin D was given and who benefitted most:
- Daily or weekly dosing without large “bolus” doses showed a clear protective effect. In this group, the adjusted odds ratio was 0.81, meaning about a 19 percent reduction in risk.
- Among people who were very deficient at the start, with vitamin D levels below 25 nmol/L, the benefit was dramatic. In that subgroup, the adjusted odds ratio was 0.30, with a confidence interval from 0.17 to 0.53. That suggests their risk of respiratory infection was cut by about 70 percent.
- People with higher baseline vitamin D levels still saw some benefit, with an odds ratio of 0.75 for those with levels at or above 25 nmol/L.
Importantly, vitamin D did not increase serious adverse events. The adjusted odds ratio for having at least one serious adverse event was 0.98, essentially no difference from placebo.
Based on these results, the authors concluded that vitamin D supplementation was safe and protective overall, and especially helpful for those who started out very deficient and who took regular daily or weekly doses instead of occasional high doses. Camargo noted that acute respiratory infections drive millions of emergency department visits in the United States, and that these findings could support efforts to fortify foods with vitamin D, particularly in populations with high deficiency rates.
For many clinicians and public health experts at the time, this analysis was seen as the strongest confirmation so far that vitamin D could help guard against colds and flu like illnesses.
Why this study was considered better than earlier research
Before this 2017 work, the field was full of mixed signals. Observational studies had linked low vitamin D levels to higher infection risk, but those studies could not prove cause and effect. People who are sicker for other reasons might stay indoors more, get less sunlight, and end up with lower vitamin D levels as a result.
Several clinical trials tried giving vitamin D supplements to see if they prevented infections. Some showed protection, others did not. Traditional meta analyses, which pool average results across trials, also produced conflicting conclusions.
The 2017 analysis improved on this in several ways:
- It used individual participant data, not just trial averages, allowing the researchers to adjust more precisely for differences between people and studies.
- It included a wide range of ages, from infants to very old adults, and people from many countries with different baseline vitamin D levels.
- It examined important modifiers like baseline vitamin D status, dosing schedule, and dose size, instead of treating all vitamin D regimens as equivalent.
Because of this, the study was viewed as a higher quality summary of the evidence, with stronger support for a protective effect, especially in people who were deficient and on steady supplementation.
However, science moves forward, and more trials kept coming. In 2021, an updated meta analysis of 43 randomized controlled trials found a modest protective effect of vitamin D against acute respiratory infections, with an odds ratio of 0.92 and a confidence interval from 0.86 to 0.99.
From one angle, the 2017 individual participant data meta analysis and its message remain important. It showed that, in carefully analyzed trials, regular daily or weekly vitamin D supplements clearly reduced respiratory infections, especially for people who started out with very low vitamin D levels. The MGH team highlighted how this could reduce emergency department visits and argued that it supports vitamin D food fortification efforts in deficient populations.
From another angle, the newer, larger 2025 meta analysis urges caution. When data from more than 64,000 participants are combined, the overall effect is small and not statistically significant. The hints of benefit in certain subgroups are interesting, but the formal tests do not confirm that those groups are truly different from the rest.
Vitamin D is important for overall health, it is generally safe at common doses, and severe deficiency is undesirable. The early meta analysis gave strong support to the idea that correcting deficiency, especially with regular daily or weekly doses, helps protect against respiratory infections. The later, larger analysis did not overturn that pattern, but it did weaken the statistical certainty and highlighted how difficult it is to prove a small protective effect across many different populations and dosing strategies.
In practice, that means vitamin D is best seen as a useful part of overall health, with possible benefits for respiratory infections that are most reliable when severe deficiency is corrected, rather than a guaranteed cure or vaccine like protection against colds and flu.
