Why is swine flu likely to return in winter? It's NOT because folks are cooped up together in winter.
Posted Jun 02 2009 4:41pm
How do these sneeze droplets differ in winter? Read on! Photo courtesy of www.dost-dongnai.gov/vn
Swine flu is so far turning out to be a milder disease than was feared a couple of weeks ago. Although it continues to spread from person to person around the globe, in most places it has been no more lethal than more familiar and common flu strains. That could very well change though when autumn rolls around.
"Our evidence from all previous pandemics is you get two phases. So you get a first wave which is often very mild and then you get a much more serious wave that comes along in the autumn and the winter," said Alan Johnson, Great Britain's Health Secretary. He was quoted May 4 in Britain's Telegraph.
All flu outbreaks tend to be worse in cold weather, and so far this H1N1 swine flu hasn't really seen any cold weather. In the Northern Hemisphere flu season is November to March. But seasons are reversed in the Southern Hemisphere - the cool weather of autumn is starting up right now in South America, where flu season is May to September.
Although there haven't been many cases of H1N1 swine flu in the Southern Hemisphere yet, we may see that change shortly. We may get a preview of what's in store for us come November.
Why are flu outbreaks always worse during the colder months? I heard an explanation on NPR's Morning Edition on Friday (5/8/09) that was completely new to me. I had always heard that flu spreads in winter because we're all cooped up together indoors. Turns out, that's not it at all!
Dr. Peter Palese of Mount Sinai School of Medicine and his colleagues have been studying the transmission of flu between guinea pigs, which can get infected with human flu viruses. Palese's results over the last couple of years indicate that spending more time together inside is not the causative factor.
Palese says flu viruses are more stable [and last longer, presumably] in colder, drier conditions. What's more, the droplets of fluid that people spray with sneezes and coughs are much smaller at lower temperatures and lower humidity, so the droplets carry much farther and stay suspended in the air longer. The smaller droplets can be inhaled more deeply into the respiratory passages and lungs, Palese says. The opposite is true at warmer temperatures and higher humidity, when the droplets become much bigger and sink to the floor, reports Palese. These influences are so pronounced that "at 75 to 80 degrees, we don't see any transmission."
Wow! Who knew! But there's even more new data to debunk the old "crowded indoors" theory.
Palese and his colleagues also report that colder, drier air makes mammals' respiratory tracts more vulnerable to infection by airborne viruses.
In cold air, "the mucous is much more viscous," Palese explains. Sticky mucus clogs up the cilia, or tiny hairs, that normally move in waves to expel virus-laden particles from the breathing passages. So in cold weather, inhaled flu viruses tend to stay where they land, reproducing and infecting the unfortunate victim.
Well, it looks like this South American winter is going to be a testing ground for the H1N1 swine flu. We'll see whether it attacks with a real vengeance, spurring the next scary flu pandemic, or whether it fizzles out like some of its predecessors have done.