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Flu mystery may be cracked

Dec. 5, 2005
World Science staff

Re­search­ers say they’ve solved a great mys­tery of the flu: why it spreads mainly in win­ter. The an­swer, they re­port, is that the vi­rus is sta­bler and re­mains air­borne long­er in the cold, dry air that pre­vails in the sea­son.

The New York Times re­ported the find­ings, al­so pub­lished in the Oct. 19 is­sue of the re­search jour­nal PLoS Path­o­gens, on Wednes­day.

A typ­i­cal flu vi­rus is shown "s­liced" in half in this di­a­gram to show the in­sides. It con­tains genes for mak­ing cop­ies of it­self on the in­side, and "spikes" that help it at­tach to a host cell on the out­side. (Cour­te­sy Nat'l In­sti­tute of Al­ler­gy and In­fec­tious Dis­eases, U.S.)


“In­fluenza vi­rus is more likely to be trans­mit­ted dur­ing win­ter on the way to the sub­way than in a warm room,” Pe­ter Palese, cha­irman of mi­cro­bi­ol­o­gy at Mount Si­nai School of Med­i­cine in New York and the stu­dy’s lead au­thor, told the Times.

Palese con­ducted the study af­ter read­ing a pa­per pub­lished in the af­termath of the dev­as­tat­ing 1918 flu pan­dem­ic. That re­port sug­gested the vi­rus could rap­idly spread among guin­ea pigs.

Palese and col­leagues ex­pe­ri­mented with flu vi­rus among guin­ea pigs, var­y­ing air tem­per­a­ture and hu­mid­ity in the an­i­mals’ quar­ters. They found that trans­mis­sion was best at 41 de­grees Fahr­en­heit (5 de­grees Cel­sius), and wors­ened at high­er tem­per­a­tures, end­ing com­pletely at 86 de­grees Fahr­en­heit (30 de­grees Cel­sius). 

Trans­mis­sion was al­so best at a low hu­mid­ity of 20 per­cent, Pa­lese said. That’s probably be­cause the vi­ruses float in the air in lit­tle res­pi­ra­to­ry droplets, he added. In hu­mid air, the droplets grow larg­er and fall.

Flu vi­ruses spread through the air, Pa­lese said, where­as cold vi­ruses spread mainly by di­rect con­tact, such as hand­shakes or con­tact with an ob­ject just touched by an in­fected per­son. Flu sea­son in north­ern lat­i­tudes runs from No­vem­ber to March, the cold­est months. In south­ern lat­i­tudes, it’s from May un­til Sep­tem­ber. In the trop­ics, there is lit­tle flu at all.

Flu re­search­ers told the Times they were de­light­ed to fi­nally get some sol­id da­ta on flu sea­sonal­ity. “It was great work, and work that needed to be done,” said Ter­rence Tumpe, a sen­ior mi­cro­bi­ol­o­gist at the U.S. Cen­ters for Dis­ease Con­trol and Pre­ven­tion.


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Researchers say they’ve solved a great mystery of the flu: why it spreads mainly in winter. The answer, they report, is that the virus is stabler and remains airborne longer in the cold, dry air that prevails in the season. The New York Times reported the findings, also published in the Oct. 19 issue of the research journal PLoS Pathogens, on Wednesday. “Influenza virus is more likely to be transmitted during winter on the way to the subway than in a warm room,” Peter Palese, chairman of microbiology at Mount Sinai School of Medicine in New York and the lead author of the flu study, told the Times. Palese conducted the study after reading a paper published in the aftermath of the devastating 1918 flu pandemic. That report suggested the virus could rapidly spread among guinea pigs. Palese and colleagues experimented with flu virus among guinea pigs, varying air temperature and humidity in the animals’ quarters. They found that transmission was best at 41 degrees Fahrenheit (5 degrees Celsius), and worsened at higher temperatures, ending completely at 86 degrees Fahrenheit (30 degrees Celsius). Transmission was also best at a low humidity of 20 percent, Palese said. That’s probably because the viruses float in the air in little respiratory droplets, he added. In humid air, the droplets grow larger and fall. Flu viruses spread through the air, Palese said, whereas cold viruses spread mainly by direct contact, such as handshakes or contact with an object just touched by an infected person. Flu season in northern latitudes runs from November to March, the coldest months. In southern latitudes, it’s from May until September. In the tropics, there is little flu at all. Flu researchers told the Times they were delighted to finally get some solid data on flu seasonality. “It was great work, and work that needed to be done,” said Terrence Tumpe, a senior microbiologist at the U.S. Centers for Disease Control and Prevention.