A dose of sunshine may soon join washing your hands and getting a vaccination on the list of things that keep the flu at bay. A recent study found more sunshine in August and September strongly protects a region from the flu bug in subsequent months.

This supports previous evidence that vitamin D created by exposure to ultraviolet radiation helps stave off respiratory tract infections. A key finding of the study is that a 10 percent increase in September’s sunny days leads to a three-point drop in a region’s 10-point flu severity index. That’s like getting a flu booster shot.

The study by the National Bureau of Economic Research combines federal data on flu incidence with a county-by-county breakdown on direct sunlight exposure in the contiguous 48 states. Researchers say this sunshine immunity is far greater than results in randomized trials where study participants received vitamin D supplements.

The benefits of sunshine are magnified in states that are not too densely populated. That’s based on a concept known as herd immunity, which means that if enough people in a group are immune to a disease, especially through vaccination, the resistance to the disease grows for everyone, even those not vaccinated.

That’s good news for folks living in sparsely populated states like Arkansas, New Mexico and Montana. People in crowded, transient states like Florida, New Jersey and California might want to double-down on the hand washing and encourage everyone to get vaccinated. Everybody gains when people look after their own health.

But don’t think sunshine can replace a flu shot, scientists say. The vaccination is still the best way to fight the virus.