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Posted at 10:30 AM ET, 05/ 6/2009

Will Summer Weather Suppress Swine Flu?

By Ann Posegate

* Full Forecast | An Imperfect Temperature Forecast *

Laboratory image of the swine flu virus. Courtesy Centers for Disease Control and Prevention.

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 Ann Posegate  | May 6, 2009; 10:30 AM ET
Categories:  Health, Posegate  
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If absolute humidity and ultraviolet light do play a prominent role in flu propagation, then we may be in a spot of trouble according to another National Geographic article.

--begin quote--
For example, ultraviolet (UV) light from the sun is not bottoming out the same way it did during the past few visual minima.
--end quote--

Mr. Q.

Posted by: Mr_Q | May 6, 2009 11:33 AM | Report abuse

Another factor involved is that influenza tends to spend the summer in nesting waterfowl populations at high latitudes. When these birds migrate southward in the fall, they transmit flu strains, which often are genetically altered by passage through the avian hosts, to swine and to humans on the ground. In fact the normal autumnal mode of transmission is waterfowl to swine to human hosts, with genetic modification also occurring in the swine host.

If this scenario holds, the current H1N1 strain will disappear this summer,and a mutated H1N1 strain of type "A" influenza will reinfect susceptible populations next fall. The new flu strain may be either milder than or more severe than the current H1N1 swine flu outbreak.

Posted by: Bombo47jea | May 6, 2009 12:57 PM | Report abuse

Actually, a new Maunder Minimum is JUST WHAT we snow-starved Washingtonians need, and will provide MUCH-NEEDED RELIEF for overworked Washington-area air-conditioning units!!!

Bring on the Little Ice Age ASAP!!! Next winter would be a great start!

Posted by: Bombo47jea | May 6, 2009 1:15 PM | Report abuse

Bring on the heat, sun, and humidity! I am in dire need of it!

Posted by: bastings | May 6, 2009 3:08 PM | Report abuse

Well, one positive way thing is that global warming will reduce deaths from influenza each year.

Posted by: RMVA | May 7, 2009 12:22 PM | Report abuse

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