Summer to Kill Swine Flu in U.S. and Mexico?

April 30, 2009

The hot and humid days of summer could prove a death knell for the swine flu outbreak currently sweeping around the globe—at least in the U.S., Mexico, and the rest of the Northern Hemisphere, experts say.

"If [the new swine flu strain] is like other types of influenza that have been tested, it would have a lower transmission rate in the summer," said Jeffrey Shaman, an assistant professor at Oregon State University in Corvallis, who studies how the water cycle affects the spread of disease.

(Related: "U.S. Swine Flu No Worse Than Seasonal Flu, Experts Say.")

Earlier this year, Shaman found that flu viruses survive longer and transmit more easily during low absolute humidity, a condition commonly associated with cold, dry winter weather.

"When we get to June, humidity rates will have increased substantially and might make for less transmission," he said.

Furthermore, in pigs—where the flu is common in the United States—outbreaks normally occur in colder months, particularly in late fall and winter, according to the U.S. Centers for Disease Control and Prevention.

Swine Flu-Susceptible Population

Though Mexico is relatively hot and humid year-round, it does have seasonal differences, Shaman noted. The dry season in México City runs from mid-December to about now.

"If this newly emerged virus developed or came into existence in México City during early March, the lower-humidity conditions then could still favor an outbreak" of swine flu, he said.

In addition, everyone may be susceptible to this H1N1 swine flu strain, because it's new. Such virulence would make an outbreak possible no matter what the climate, Shaman noted. Higher absolute humidity might only slow the swine flu's spread.

Georges Benjamin, executive director of the American Public Health Association, added that the new swine flu virus may "peter out as it gets warmer," especially now that we're at the end of the traditional flu season.

Summer Slowdown

Though transmission of swine flu could decrease in the months ahead, it won't simply disappear, Oregon State's Shaman added. The question is whether the post-summer H1N1 swine flu virus will be worse, milder, or just barely noticeable?

"These things, unfortunately, really depend on the biology of the virus."


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