Feb 3, 2011 (CIDRAP News) – Aerosolized flu viruses, the type that can remain airborne for hours, circulate in places such as doctor's offices, daycare centers, and airplanes in doses large enough to cause infection, researchers from Virginia Tech reported yesterday.
The researchers described their findings in a report scheduled to be published in the Journal of the Royal Society Interface. The team included Linsey Marr, PhD, associate professor of civil and environmental engineering, virologist Elankumaran Subbiah, PhD, and graduate student Wan Yang.
The study, along with two other recent ones on aerosol flu transmission, could shed more light on the role of the smallest particles. The strongest evidence on flu spread has shown that larger respiratory droplets, those dispersed a short distance and over a short time around an infected person, are a major driver in flu transmission.
However, some experts have suspected that airborne particles also play an important role, based on infection patterns in environments such as airplane cabins. Controversy about the role of each flared during the 2009 H1N1 pandemic as health officials sought to craft the best evidence-based guidance for protecting healthcare workers with masks, N-95 respirators, and other protective equipment.
Marr said in a press release that assessing the role of aerosolized particles in the spread of flu is difficult, because it's difficult to involve humans in the studies and hard to detect influenza A viruses in ambient air.
To assess the concentration of flu viruses in public places, the team collected air samples from the waiting room of a health center at Virginia Tech, a daycare facility in Blacksburg, Va., and commercial airline flights between Roanoke, Va., and San Francisco.
Using a cascade impactor, a sampling device that allows researchers to separate biologic aerosols into five different size ranges, they collected 16 samples between Dec 10, 2009, and Apr 22, 2010. Then they used quantitative reverse-transcriptase polymerase chain reaction (qRT-PCR) to detect genetic evidence of influenza A viruses.
"Size is important because it determines how long the particles will remain suspended in the air before being removed due to the forces of gravity or other processes," Marr said.
Half of the air samples contained aerosolized flu viruses: 3 of 9 from the health center, 3 of 4 from the daycare center, and 2 of 3 from airplanes.
When the researchers looked at the amounts of virus for each of five particles sizes, they found that 36% of the viral material was associated with particles larger than 2.5 micrograms (mcg), 28% was linked to those between 1 and 2.5 mcg, 11% to those between 0.5 and 1 mcg, 10% to those between 0.25 and 0.5 mcg, and 15% to those smaller than 0.25 mcg. They didn't find any obvious particle size distribution patterns in the eight positive samples.
In estimating airborne influenza virus concentrations, the team found a larger fraction of total genome copies to be associated with fine particles: 80%with particles smaller than 2.5 micrograms.
Marr said the average concentration was 16,000 viruses per cubic meter of air and that the majority of viruses were fine particles. "Given those concentrations, the amount of viruses a person would inhale over 1 hour would be adequate to induce infection," she said.
The authors found that the average aerosol flu virus concentration was two times higher at the daycare center than at the health center. "Considering that children are the primary susceptible population of influenza, the difference is not surprising," they wrote.
From the daycare samples they observed that the largest particles contained a larger portion of flu genome copies than was the case at the health center, which they said might relate to the stage of infection. Children are sent home as soon as symptoms appear, but people at health centers are often symptomatic. The virus-containing particles in the airplane samples were evenly distributed across the different sizes, which might suggest a more diverse age range, the report says. It adds that the possible relationships need further investigation.
The researchers cautioned that though qPCR is a good tool for detecting viral genetic material, it doesn't reveal whether the virus is viable. They added that another important research question to explore is whether viruses are equally viable across different particle sizes.
Also, they said the exposure doses, which they said were adequate to cause infection, provide only a rough estimate of amount inhaled. Fore example, they don't take into account clearing action in the respiratory tract.
Concerning virus deposition on surfaces, the investigators said that though the findings suggest that all surfaces, not just ones touched by patients, can harbor viruses, the probable amount seems unlikely to pose an infectious threat and the viruses aren't thought to survive longer than 2 or 3 days.
Lisa Brosseau, ScD, an industrial hygiene expert from the University of Minnesota School of Public Health who has studied respiratory protection for several years, told CIDRAP News that the authors' and protocol seem appropriate. She added that the finding that a significant portion of virus-laden particles occur at sizes smaller than 2.5 micrograms, the ones that can remain airborne for a long period, supports the hypothesis that flu transmission could occur from exposure to small particles at close range.
She said the modeling of likely airborne concentrations also seems solid and suggestive of close-range aerosol flu transmission, with the authors carefully addressing the limitations.
Dr Raymond Tellier, MD, MSc, a flu transmission expert and a microbiologist with the Provincial Laboratory for Public Health in Calgary, Alta., said the study is an important contribution that will help clarify controversial questions about flu spread.
He said the study design was thorough and rigorous, with the findings consistent with recently published studies by Blachere and colleagues in 2009 and Lindsley and colleagues in 2010. He commented that the study is also useful because it extends the findings to settings beyond healthcare facilities.
"Particularly welcome also is a thorough analysis of the infectious risk unearthed by these observations, and a discussion pointing to the negligible risk of surface contamination by the aerosolized viruses," Tellier said.
Yang W, Elankumaran S, Marr LC. Concentrations and size distributions of airborne influenza A viruses measured indoors at a health center, a daycare, and on airplanes. J Royal Soc Interface 2011Feb 2 [Journal homepage] (Editor's note: The study was scheduled for publication Feb 2 but had not yet been posted at this writing.)
Feb 1 EurekAlert press release