Until now, most people thought you could catch the flu after being exposed to droplets from an infected person’s coughs or sneezes, or by touching contaminated surfaces.
But a new study released Thursday in the Proceedings of the National Academy of Sciences reveals that we may pass the flu to others just by breathing.
The study — which included researchers from San Jose State University and UC Berkeley — provides new evidence for the potential importance of the flu’s airborne transmission because of the large quantities of infectious virus researchers found in the exhaled breath from people suffering from flu.
“The study findings suggest that keeping surfaces clean, washing our hands all the time, and avoiding people who are coughing does not provide complete protection from getting the flu,” Sheryl Ehrman, dean of the College of Engineering at San Jose State University, said in a statement.
“Staying home and out of public spaces could make a difference in the spread of the influenza virus.”
The study was done at the University of Maryland during the flu season of December 2012 through March 2013. Researchers there recruited 178 volunteers, mostly students, who were within the first three days of the flu’s onset.
Over four months, researchers captured and characterized the flu virus in exhaled breath from 142 of the volunteers who had confirmed cases of the flu, as the volunteers breathed naturally, talked, coughed or sneezed.
The researchers then assessed the infectivity of naturally occurring flu aerosols, tiny droplets that stay suspended in the air for a long time.
The study said participants provided 218 swabs from the upper part of their throats that lies just behind the nose, and the same number of 30-minute samples of exhaled breath, spontaneous coughing, and sneezing on the first, second, and third days after the onset of flu symptoms.
The analysis of the infectious virus recovered from these samples showed that a significant number of flu patients routinely shed infectious virus, not merely detectable ribonucleic acid, or RNA, into aerosol particles small enough to present a risk for airborne transmission.
Surprisingly, the study said, 11 of the 23 fine aerosol samples acquired in the absence of coughing had detectable viral RNA, and 8 of these 11 contained infectious virus, suggesting that coughing was not necessary for infectious aerosol generation in the fine aerosol droplets.
In addition, the researchers said, the few sneezes observed were not associated with greater viral RNA copy numbers in either coarse or fine aerosols, suggesting that sneezing does not make an important contribution to influenza virus shedding in aerosols.
“We found that flu cases contaminated the air around them with infectious virus just by breathing, without coughing or sneezing,” Dr. Donald Milton, professor of environmental health in the University of Maryland School of Public Health, said in a statement.
“People with flu generate infectious aerosols even when they are not coughing, and especially during the first days of illness,” he said. “So when someone is coming down with influenza, they should go home and not remain in the workplace and infect others.”
The researchers believe that their findings could be used to improve mathematical models of the risk of airborne flu transmission from people with flu symptoms. The results could help develop more effective public health interventions and to control and reduce the impact of influenza epidemics and pandemics.
Improvements also could be made to ventilation systems to reduce transmission risk in offices, school classrooms and subway cars, for example, the study said.
For now, the researchers — and public health experts — say everyone should heed the advice to stay home, if possible, when they’re starting to come down with the flu to prevent the virus from spreading.
And while getting a flu vaccine isn’t a guarantee that you won’t get the flu, experts say it provides some protection and helps reduce the chances of someone becoming seriously ill from the flu.
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