Influenza infections follow distinct patterns around the world, and a new study links some of these changes to the local climate.
In particular, researchers found a link between humidity and the time of year when infections peak, which one day could help health officials plan for seasonal outbursts as well as infection changes from a warming planet. They published their findings last week in the journal PLOS Pathogens.
Looking at a patchwork of influenza data and climate records gathered from 78 sites around the world, the researchers pieced together a picture of how humidity causes changes in seasonal influenza. "This study has just been made possible because we have been able to gather influenza surveillance data" from around the world, explained co-author Cécile Viboud, a staff scientist at the Fogarty International Center at the National Institutes of Health.
Eventually, a trend emerged. "We found there were two distinct seasonal patterns of influenza around the world," Viboud said. In temperate regions, influenza surges in the winter, when the air is cold and dry. Closer to the equator, the illness tends to spread year-round, with peaks during rainy seasons.
In fact, researchers uncovered a threshold where influenza shifts from one transmission pattern to the other: Sites that experienced temperatures below 18 to 21 degrees Celsius and specific humidity below 11 to 12 grams per kilogram for at least one month experienced seasonal influenza epidemics during cold, dry months. Above this line, regions experienced high influenza rates when they were drenched.
Now there's a link, but what's the cause?
Researchers aren't certain why humidity tends to lower influenza transmission in some environments while increasing infections in other locales, but they have some ideas. "There have been animal experiments that show that cold temperatures and low humidity favor influenza activity," Viboud said.
This would explain influenza trends in cooler parts of the world. "It's been hypothesized that in temperate locations ... that airborne transmission could increase" during winter, said James Tamerius, the study's lead author and an Earth Institute postdoctoral researcher in the environmental health sciences department at Columbia University. "But it's unclear why higher levels of humidity or precipitation would increase it."
One proposed mechanism is that in warmer climates, rainy weather may force people indoors in close proximity, increasing the chances of person-to-person transmission, according to Viboud. Tamerius observed that vacationers to sunny tropical beaches might also unwittingly bring influenza from other regions, causing cycles of local outbreaks.
As the world gets warmer, these transmission patterns may shift so that influenza rises during different months. "For instance, in places that used to have a very large rainy season, then we could see influenza pop up in other times of the year," Viboud said, adding that the overall number of infections is likely to remain unchanged.
However, the scientists said it is still too early to include influenza risk along with your weather forecast. "It's very hard to make any predictions at this time," Tamerius said, citing the need for more laboratory experiments as well as better information about human behavior patterns in relation to vaccination rates and susceptibility.
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