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Determining the modes by which the influenza virus can be transmitted is more than just an academic exercise for researchers, as the answers will help determine how we protect front line health care workers, patients, and the general public during a pandemic.
The `classic’ answer is that influenza is spread primarily by `large droplets' expressed by infected individuals when they cough or sneeze. These heavy viral laden droplets are assumed to travel only a few feet, and to settle to the ground (or other surfaces) in a matter of seconds.
Photo Credit PHIL (Public Health Image Library)
A secondary route of infection, via fomites (inanimate objects that have viral contamination) is also assumed, although how much fomites contribute to influenza transmission is unknown.
But the third possibility, aerosolized particles that can remain suspended in the air for minutes or even hours, and potentially travel much longer distances, remains controversial.
Last November in Study: Aerosolized Transmission Of Influenza, we saw a report of an influenza patient receiving a ventilation treatment in an open ward next to another air current producing device that lead to the apparent aerosolized spread of the virus.
Last September in Ferreting Out The Transmissibility Of Aerosolized H5N1 we saw a study on the transmission of aerosolize bird flu among ferrets.
And in August of last year, in Another Mask Study To Ponder, we looked at research on the relative effectiveness of N95 respirators and surgical masks in blocking aerosolized particles.
Today, a press release on study to be published today in the Journal of the Royal Society Interface that looked at the amount of influenza virus suspended in the air in several environments, including a daycare center, a healthcare waiting room, and aboard commercial aircraft.
They found airborne virus particles in half of the air samples tested, and in quantities they believe sufficient to enable transmission of the virus.
Many of these virus particles were very small, less than 2.5 micrometers, which can remain aloft on a room’s air currents for hours. Larger droplets would settle far sooner, and theoretically present less of a threat.
So size, in this case, really matters.
Size of airborne flu virus impacts risk, Virginia Tech researchers say
A parent's wise advice to never go to a hospital unless you want to get sick may be gaining support from scientific studies on a specific airborne virus.
The results of a Virginia Tech study by environmental engineers and a virologist on the risk of airborne infection in public places from concentrations of influenza A viruses is appearing today in the on-line, Feb. 2 issue of the United Kingdom's Journal of the Royal Society Interface.
Linsey Marr, associate professor of civil and environmental engineering at Virginia Tech, http://www.cee.vt.edu/people/lmarr.html and her colleagues, Wan Yang, of Blacksburg, Va., one of her graduate students, and Elankumaran Subbiah, a virologist in the biomedical sciences and pathobiology department of the Virginia-Maryland Regional College of Veterinary Medicine, http://www.vetmed.vt.edu/org/dbsp/faculty/subbiah.asp conducted their research in a health center, a daycare facility, and onboard airplanes.
"The relative importance of the airborne route in influenza transmission—in which tiny respiratory droplets from infected individuals are inhaled by others—is not known," Marr, who received a National Science Foundation CAREER Award to pinpoint sources of unhealthy air pollutants, said.
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To conduct their studies, the Virginia Tech researchers collected samples from a waiting room of a health care center, two toddlers' rooms and one babies' area of a daycare center, as well as three cross-country flights between Roanoke, Va., and San Francisco, Ca. They collected 16 samples between Dec. 10, 2009 and Apr. 22, 2010.
"Half of the samples were confirmed to contain aerosolized influenza A viruses," Marr said. "In the others, it is possible that no infected individuals were present."
Marr added, "The average concentration was 16,000 viruses per cubic meter of air, and the majority of the viruses were associated with fine particles, less than 2.5 micrometers, which can remain suspended for hours. Given these concentrations, the amount of viruses a person would inhale over one hour would be adequate to induce infection."
Although the article has not yet shown up in the journal, you should be able to access the abstract later today HERE.
While there remain questions on the viability of these viral particles, and of the viral load required to produce infection, the authors found:
"As a whole," the three authors concluded in the Journal of the Royal Society Interface, "our results provide quantitative support for the possibility of airborne transmission of influenza in indoor environments."
No definitive answers here, of course.
But more data to be taken into account when making policy decisions regarding the proper kinds of PPEs (Personal Protective Equipment) and social distancing recommendations during a pandemic.
