Vitamin D, something that your immune system uses to produce antimicrobial deterrents, may be the key to fighting off colds and influenza viruses. According to a review published in the British Medical Journal, if it formed a more major part of the diet of those in the UK, up to 3 million people could avoid falling ill every single year.

This is being widely reported as a proverbial miracle of modern medical research, but there have been plenty of warnings before about taking unnecessary dietary supplements and the harm they can cause you. So what’s the deal this time?

Recent analyses have placed Vitamin D supplements as being anywhere from “promising” to “inconclusive” – essentially, there’s no strong evidence that taking them, if you’re a perfectly healthy individual, will make any difference to your biological workings.

Most people, particularly those in Northern Europe, do not get enough sunlight in the winter months, which is required by the body to generate Vitamin D. Low levels of this particular chemical compound correlates with a weaker immune system, so it’s been thought for some time that those lacking it would be more likely to be successfully infected by pesky viruses in the winter months.

Multiple trials on using supplements to prevent such infections have been conducted many times before, but the results have never been clear-cut, and medical professionals are wary about overselling the efficacy of potential treatments to patients in need.

This new review, led by the Queen Mary University of London (QMUL), looked at 11,321 people across 25 separate trials in order to see if there was any definitive correlations present in this regard. These studies looked at an impressive range of infections, from common cold strains to full-blown influenza.

They came to the conclusion that, for every 33 people regularly taking Vitamin D supplements as part of a balanced diet, one of them would not experience a cold or flu infection during the year. This would make it more effective than the flu vaccination.

Overall, extended to the entire UK, this works out to be 3 million people without the sniffles. This would not just be great for their own health, but would be a huge boost to the British economy in terms of work hours not lost to sickness.

However, before leaping to the nearest pharmacy to ask for vitamin supplements, it’s worth looking at the accompanying editorial in the journal. It points out that within just three months, two studies – this one, and another earlier one – have come to very different conclusions, with one concluding that vitamin D has no effect on disease prevention, with the exception of muscoskeletal diseases.

“Given the short time between articles, why are the conclusions so different?” the editorial reads, adding that the authors of it think that “there are reasons for viewing the headline result cautiously.”

“In absolute terms, the primary result is a reduction from 42% to 40% in the proportion of participants experiencing at least one acute respiratory tract infection,” it notes. This means that taking Vitamin D supplements reduces your risk of getting a cold by 2 percent, which is very minor indeed.

The editorial also points out that the term “acute respiratory tract infection” is loosely defined throughout the 25 trials, meaning that it’s not really clear what Vitamin D supplements are actually having an effect on.

“Should these results change clinical practice? Probably not,” it concludes. Right now, the supplement hypothesis is just that – a hypothesis – and one that needs plenty more randomized and controlled trials to be confirmed.

So, for now, Vitamin D supplements remain in the “inconclusive” column. Don’t believe the hype just yet.