Cures and remedies for the common cold have been bandied about for millennia, from mustard plasters and steam baths to chicken soup and sassafras tea. But only a few of them have  really done more than provide a modicum of relief, whether from placebo affect or an actual biological process.

To try and find out what (if anything) can truly help alleviate the cruel effects of colds and flus, a group of scientists analyzed over 25 randomized controlled trials that covered over 11,000 people in the U.S., U.K., and Canada. In the meta-study, which was published in The BMJ, the researchers looked at individual participants in the trials—a much more detailed glimpse at the relevant information.

They found that for those who were deficient in vitamin D, daily or weekly supplementation gave them the most value and ended up slashing their risk of getting an acute respiratory infection by 50%. And the researchers were also able to show that all participants—not just the deficient ones—gained some benefit from regular vitamin D supplementation, though random high doses didn’t do much, if anything to help.

“Our analysis has also found that [vitamin D] helps the body fight acute respiratory infection, which is responsible for millions of deaths globally each year,” said senior study author Carlos Camargo, M.D., Dr.PH., a professor of emergency medicine at Harvard Medical School. “These results could have a major impact on our health system and also support efforts to fortify foods with vitamin D, especially in populations with high levels of vitamin D deficiency.”