By Jamie Ducharme
March 2, 2018
TIME Health
For more, visit TIME Health.

By now, you probably know the tried-and-true flu prevention strategies: get vaccinated, wash your hands often and try to steer clear of folks who are sick. But new research says there may be another way to stay healthy, and it begins before flu season is even in full swing.

Spending more time in the sun in August and September, and thus absorbing more immune-system-boosting vitamin D, may help prevent the flu as fall progresses, according to a new working paper distributed by the National Bureau of Economic Research. A state that sees an unusually high number of sunny days in a given month, the paper says, can also expect to see an unusually low number of flu cases during that time period.

The largest reduction is seen in September (and October, to a lesser extent), when sunlight and influenza are both relatively common, explains David Slusky, an assistant professor of economics at the University of Kansas who co-authored the paper with Harvard Kennedy School political economy professor Richard Zeckhauser.

The sun provides some flu protection throughout the year, Slusky says, “But if you test each month one at a time, the only one of statistical significance is really September. You need to have enough sunlight and flu variation. That really only happens in the early fall, late summer.”

Using flu data from the Centers for Disease Control and Prevention (CDC) and weather data from the North American Land Data Assimilation System, the researchers calculated that if a state saw a 10% increase in September sunlight, it could also expect a three-point drop on the CDC’s flu severity index for that month. And while September may not be the peak of flu season, the paper says a decrease that significant would have meant roughly 29,500 fewer cases of the flu over the course of Septembers 2009, 2010 and 2011.

The effect may be especially pronounced in states with moderately dense populations, according to the research. That’s thanks to a concept called herd immunity, which assumes that a high level of immunity in a population will keep individuals from getting sick. Herd immunity doesn’t matter much when a state’s population is extremely spread out, and very crowded states may never have low enough levels of disease to reap the full benefit. States in the middle, however, may be well placed to benefit. These include Georgia, Indiana, Kansas, Kentucky, Missouri, New Hampshire, New Mexico, North Carolina, Tennessee, Arizona, Colorado, Michigan, Minnesota, Nebraska, Ohio, Texas, Utah and Wisconsin, according to the paper.

Of course, there’s no way to control how many sunny days your state sees in a given month. But the research suggests that taking advantage of however much sunlight it does get, and therefore boosting your vitamin D levels, may do you some good, Slusky says.

“There’s a sweet spot where you’ll get your vitamin D but you won’t get sunburned,” Slusky says. “There’s evidence that vitamin D reduces upper respiratory infections, and you’ll get more vitamin D from getting more sunlight.”

Vitamin D helps the immune system fight off bacteria and viruses, so it stands to reason that getting more of it could help prevent illnesses such as the flu. Studies have also shown that taking vitamin D supplements can cut your risk of respiratory illness. Vitamin D supplements, however, aren’t without risk, and direct exposure may offer the same benefits. Since the vitamin lives in your fat stores for up to two months at a time, stockpiling sunshine during the late summer and early fall could theoretically keep you healthy as flu season heats up.

But while soaking up a few rays may do your body good, Slusky emphasizes that it’s not a replacement for vaccines and other flu prevention measures. “This is not a substitute for the flu shot. This is not a substitute for washing your hands,” he says. “These things are complements and work together.”

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