The Doctor Weighs In

By Dov Michaeli MD, Ph.D.

When I was a young kid I used to annoy my dad with incessant questions of Why? “Why is the sky blue”? He couldn’t give me a satisfactory answer, and I had to go to college to find the answer. Now it’s payback time: my granddaughter is bombarding me with daily questions, like “why do men wear pants and women wear skirts”? and other such questions for which there are no good straightforward answers.

Here is a question I kept asking myself (in preparation for Rebecca’s inevitable question) and for which I didn’t have a satisfactory answer: why do the flu epidemics occur only in the winter? To my delight I discovered at least a partial answer in the March 2 of the online edition of Nature Chemical Biology.

Will a leopard change its spots?

When it comes to viruses, the answer is a definite maybe. A team of NIH scientists used a wonderfully named technique called magic angle spinning nuclear magnetic resonance to create detailed images of how the virus's outer membrane responded to variations in temperature. The virus's outer membrane is composed chiefly of lipids. As we discovered from deep observations in the kitchen, butter is solid in the refrigerator, gets softer at room temperature, and turns liquid in the frying pan. Dr. Zimmerberg and his colleagues found that at temperatures slightly above freezing, the virus's lipid covering solidified into a gel. When temperatures reached about 60 degrees Fahrenheit, the outer membrane began to thaw, and finally turned to a soupy mix.

Cooler temperatures, apparently, cause the virus to form the rubbery outer covering that can withstand travel from person to person. Once in the respiratory tract, the warm temperature in the body causes the covering to melt to its liquid form, so that the virus can infect the cells of its new host.

In the winter, the lipid covering protects the virus from destructive environmental conditions when it travels in the fomites (small droplets formed when we sneeze or cough). In spring and summer, however, the temperatures are too high to allow the viral membrane to enter its gel state. The individual flu viruses apparently dry out and weaken, and the flu season wanes.

What are the implications?

I remember my old folks warning me not to stay in a hot room and then go out into the cold: You’ll get the ‘grippe’ (French and Yiddish for the flu) was their anguished cry as I ran out to play on the street. Well, they were only partly right. Yes, when temperatures in the winter dropped to the 50’s and 40’s, I was much more prone to get the flu (accompanied by the inevitable reproach of “I told you so”). But the heated room? This is exactly what is needed for protection against transmission of the flu virus. As the authors of the study theorized, in areas affected by a severe form of the flu, people might better protect themselves against getting sick by remaining indoors at warmer temperatures than usual.  Let me add my two cents' worth: get a flu shot!

My take

Sophisticated study that for the first time offers a plausible explanation for the winter flu epidemics. But still…

How do you explain flu epidemics in Florida ? Can we blame it on the ‘Canadian geese’ migrating from the Northeast?

Many other viruses contain lipids in their envelope, yet their spread is not restricted to winter.

There must be other factors that account for the puzzling phenomenon of winter epidemics, but this study is an important step forward in solving the puzzle.

Yes Rebecca, I have the answer, in case you wondered.