Vitamin D supplements could spare more than three million people from colds or flu in the UK each year, researchers claim, BBC reports.
The sunshine vitamin is vital for healthy bones, but also has a role in the immune system.
The analysis, published in the British Medical Journal, argues food should be fortified with the vitamin.
But Public Health England (PHE) says the infections data is not conclusive, although it does recommend supplements.
These, it says, should be taken for improved bone and muscle health.
The immune system uses vitamin D to make antimicrobial weapons that puncture holes in bacteria and viruses.
But as vitamin D is made in the skin while out in the sun, many people have low levels during winter.