Dec 2, 2010 (CIDRAP News) The role of aerosols in the transmission of flu viruses has been unclear and controversial, but a new study that analyzed the size and content of particles from the coughs of flu patients found that particles small enough to remain airborne can contain detectable flu virus.
The study found that the viral RNA in the particles can remain airborne and thus could be inhaled deep into the lungs, which poses infection control challenges in health facilities. The study, conducted by investigators from West Virginia University, appeared Nov 30 in Public Library of Science (PLoS) One.
Aerosol versus droplet transmission of flu became a hot-button issue during the 2009 H1N1 pandemic as federal officials, professional groups, and hospitals struggled to craft science-based recommendations for respiratory protection for healthcare workers. Contributing to the debate was the concern that airborne viruses, if drawn deep into the lungs, might lead to more severe illness.
During the H1N1 pandemic the US Centers for Disease Control and Prevention (CDC) recommended N-95 respirators for all healthcare workers in close contact with flu patients, but many opposed the advice on the basis that respirators are uncomfortable to wear for long periods of time. In June, when the CDC revised its flu guidance for healthcare facilities, it relaxed some of its respiratory protection advice, urging healthcare workers to wear surgical masks when caring for confirmed and suspected flu patients and to wear N-95 respirators or higher levels of protection during risky tasks such as aerosol-generating procedures.
The West Virginia investigators recruited people with flu symptoms who sought care at a student health clinic in October and November 2009 during the second H1N1 pandemic wave. Subjects were given a rapid flu test, and two nasopharyngeal samples were collected from each of them for PCR and viral plaque assay testing. Researchers took their temperatures and obtained brief health histories.
Overall, influenza was detected in 47 of 58 volunteers. To collect cough samples, researchers used a National Institute for Occupational Safety and Health (NIOSH) two-stage bioaerosol cyclone sampler or an SKC BioSampler. The NIOSH unit collects particles in dry tubes with filters, and the SKC device collects particles in liquid.
The team used the NIOSH sampler to collect cough aerosols from 48 of the volunteers, 38 of whom tested positive for flu, and the SKC BioSampler to collect aerosols from 10, of whom 9 tested positive for flu.
RNA was detected in coughs from 38 (81%) of the 47 subjects who had flu. The RNA was found in 35% of particles larger than 4 microns in diameter, 23% of particles 1 to 4 microns in diameter, and 42% in particles smaller than 1 micron. Viable virus was found in aerosols from 2 of 11 volunteers who had viable virus on nasopharyngeal swabs.
The investigators noted that the fraction of respirable particles that contained influenza RNA, 65%, was higher than the 42% to 53% range found in previous aerosol sampling studies that were conducted in healthcare facilities.
When the researchers compared the amount of viral RNA in the nasopharyngeal samples and the cough-generated aerosols, they weren't surprised when findings suggested that patients with higher viral loads from the nasopharyngeal region shed more viral RNA during coughing, but they said the variation they found from patient to patient was notable. "In fact, 45% of the influenza viral RNA from cough aerosols collected using the NIOSH sampler came from just 4 of 38 subjects with influenza," they wrote, adding that the findings suggest some flu patients may be "superspreaders."
The researchers said the results show that coughing flu patients emit aerosols that contain flu virus and that respirable particles (smaller than 4 microns) contain much of the viral RNA.
The group concluded that in some cases, coughing flu patients release airborne particles that contain potentially infectious virus, which they say supports the view that aerosol transmission is possible. However, they noted that other factors, such as virus survival time and infectivity of airborne viruses, remain unclear.
Lisa Brosseau, ScD, MS, an industrial hygiene expert at the University of Minnesota who has studied respiratory protection for years, told CIDRAP News that the study appears solid and used up-to-date sampling and analytic methods.
She said the findings add important information about the size distribution of infectious aerosol particles from coughs. "While other studies have demonstrated that individuals with influenza can generate particles in a wide range of particle sizesand that many particles are in a relatively small particle size (less than 1 micron) that can remain airborne for long periods of timeI believe this is one of the first studies to demonstrate the viability of such particles," Brosseau said.
She said she has argued for some time that current infection control paradigms ignore the role of aerosol transmission involving small particles generated near, for example, coughing patients.
"It is much more likely that these smaller particles, rather than the larger droplet splashes argued by many, are responsible for person-to-person transmission," she said. "Smaller particles are easily inhaled and have a high probability of depositing in the lower alveolar regions of the lung."
The study supports an influenza transmission route that includes near-range exposure to small infectious aerosol particles, Brosseau added.
Lindsley WG, Blachere FM, Thewlis RE, et al. Measurements of airborne influenza virus in aerosol particles from human coughs. PLoS One 2010 Nov 30 [Full text]
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