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Posted at 10:30 AM ET, 05/06/2009

Will Summer Weather Suppress Swine Flu?

* Full Forecast | An Imperfect Temperature Forecast *

Bring on the heat -- and the humidity. Bad hair days may be just what the doctor ordered for slowing the spread of swine flu.

Traditional flu season typically occurs during the coldest and driest time of the year, usually November through April in the Northern Hemisphere, before petering out. Will swine flu follow a similar trend and slow or stop its spread this summer? Possibly.

What is it about summer that foils the flu virus?

Keep reading for more on what summer could mean for the swine flu...

Scientists have suggested various theories to explain why flu is most prevalent during winter. One is that cold weather weakens our immunity to the influenza virus, a theory that does not have strong evidence behind it. Another is that we spend more time indoors and come into close contact with others more often in winter. Yet another is that the mucous lining our respiratory tracts -- a natural defense against germs -- is drier in winter, giving the virus easier entry into cells.

However, recent studies have shown that the main cause is weather: the virus itself survives better in cold, dry conditions.

Scientists believe that warm temperatures melt the virus's outer coating and that moisture in the air prevents the virus from lingering in the air (they aren't sure why moisture has this effect). In 2007, researchers at Mount Sinai School of Medicine in New York determined that low temperatures and low relative humidity help the virus survive longer and spread more easily. The study showed that survival and transmission of the virus in guinea pigs is strongest around 41 degrees Fahrenheit and 20 percent relative humidity. The virus did not spread at all in temperatures at or above 86 degrees and relative humidities of 80 percent or more.

A more recent study at Oregon State University concluded that absolute humidity affects the transmission of influenza virus even more so than relative humidity. (Relative humidity represents the amount of moisture in the air compared to the maximum amount of moisture that could exist in the air at a given temperature; absolute humidity is the actual amount of moisture in the air regardless of temperature.) A similar but stronger trend was found: the influenza virus has a better chance of survival and transmission when exposed to low absolute humidity.

In the Northern Hemisphere, swine flu may follow suit and die out as summertime heat and humidity settle in. But Dr. Ann Schuchat, interim deputy director for the Centers for Diesase Control and Prevention, warns that the case is not cut and dry.

"Every influenza expert that I hear from really cautions me in anything I say to remind people how unpredictable influenza can be -- especially a new strain. So we do optimistically hope things will get better because of the season but we need to be attentive for the fall afterwards" when swine flu could reemerge, said Schuchat in a congressional hearing last week.

Some scientists suggest that getting adequate amounts of vitamin D, which our bodies create when exposed to UV-B rays from the sun, can boost our immunity to upper respiratory tract infections, including flu. In general, people outside the tropics spend less time outdoors in winter and thus have less exposure to sunlight. Vitamin D deficiency can result, making us more susceptible to the virus (related article).

For now, all we can do is take precautions to avoid catching swine flu, try not to fall victim to the many flu myths, and wait for whatever weather this summer has in store...

Weather Underground's Jeff Masters has more on weather and the flu.

By  |  10:30 AM ET, 05/06/2009

Categories:  Health, Health

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