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Yes, says a study published on June 14 in the open access online medical journal Plos One. Make absolutely certain you have higher-than-normal blood levels of vitamin D.

Here's what you need to know about the study and its practical application.

The study was led by James R. Sabetta, MD, in the Department of Medicine at Yale University School of Medicine and the Section of Infectious Diseases at Greenwich Hospital in Greenwich, Connecticut. He and his colleagues observed the obvious: rates of "acute respiratory tract infections" (colds and flu) rise in the fall and winter. But why? Could the seasonal drop in blood levels of vitamin D -- a hormone-like nutrient produced most abundantly in the body when the skin is exposed to the strong, direct sunlight of summer -- explain the phenomena?

To find out, the researchers took monthly measurements of the blood levels of vitamin D (25-hydroxyvitamin D) in 195 healthy adults. The measurements started the third week in September and continued for the next four to five months. At the same time, the study participants were asked to report any acute respiratory tract infections. The results were, well, decisive.

Those who had blood levels lower than 38 ng/ml (nanograms per milliliter) had twice as many upper respiratory tract infections.

Among the 18 people in the study who consistently maintained blood levels of vitamin D above 38 ng/ml, 15 were completely free of upper respiratory tract infections -- no colds, no flu! (Of those 18 folks, 13 were taking vitamin D supplements. More about that in a minute.)

And when the above-38 group did succumb to cold or flu, their illnesses were shorter. The percentage of days ill with acute respiratory tract infections in the above-38 group was 4.9 times lower than in the below-38 group.

Of the other 180 participants -- all of them with blood vitamin D levels consistently below 38 ng/ml -- 81 developed colds and flu.

The study's statistical summary: the 38 plus group had a two-fold decrease in the risk of developing a cold or flu.

The Yale researchers aren't the first to link vitamin D levels and the flu. In research reported in the American Journal of Clinical Nutrition in March, 2010, Japanese doctors studied 334 children, half of whom took 1200 IU of vitamin D daily. Eighteen of the children taking vitamin D developed the flu, compared to 31 children not taking the vitamin, a risk reduction of 58 percent. An earlier study in the Archives of Internal Medicine looked at 19,000 adults and adolescents and found that those with the lowest blood levels of vitamin D were 40 percent more likely to have had a recent cold or flu, compared to those with the highest levels. In another study, women taking 2,000 IU of vitamin D (to protect bones) had an average of nine episodes of colds and flu over three years of taking the supplement -- compared to an average of 30 episodes in a group of women taking 200 IU of vitamin D.

But the Yale study was the first to methodically track vitamin D levels and colds/flu incidence during the cold/flu season. What did the Yale researchers have to say about their startling results?

"Maintenance of a 25-hydroxyvitamin D serum concentration of 38 ng/ml or higher should significantly reduce the incidence of acute viral respiratory infections and the burden of illness caused thereby, at least during the fall and winter." Easier said than done.