Patients Emit Influenza Virus Aerosols Up to 6 Feet Away

Daniel M. Keller, PhD

November 01, 2012

SAN DIEGO, California — Healthcare workers may be exposed to infectious amounts of influenza virus in small-particle aerosols as far as 6 feet from patients. Speaking here at IDWeek 2012, Werner Bischoff, MD, PhD, said that "super emitters" — patients releasing influenza virus in particles in exceptionally high concentrations — show that infection transmissibility may vary among patients.

The US Centers for Disease Control and Prevention and the World Health Organization currently recommend fit-tested respirator masks only during aerosol-generating procedures such as bronchoscopies because influenza transmission is thought to occur mainly by large-particle respiratory droplets that face masks can block.

However, Dr. Bischoff, assistant professor of internal medicine in infectious diseases and epidemiology and prevention at Wake Forest School of Medicine and assistant director of hospital epidemiology at Wake Forest University Baptist Medical Center in Winston-Salem, North Carolina, and colleagues recognized a need for research into its transmission in light of the potential severity of influenza epidemics in different years.

During the 2010-2011 influenza season, they studied the spatial distribution of influenza aerosols generated by symptomatic patients in their tertiary care teaching hospital to identify features associated with the release of high levels of particles containing the virus. The hospital has a policy of mandatory influenza vaccination for all healthcare providers.

The researchers obtained nasopharyngeal swabs for rapid bedside influenza testing from patients older than 2 years who were admitted to the emergency department (ED) or to an inpatient care unit with influenzalike illness (ILI), characterized by fever higher than 37.8° C or a patient report of cough or sore throat and suspected influenza within the past 12 hours.

The investigators enrolled 94 patients with ILI symptoms in the study, of whom 65% were positive for influenza (n = 31 influenza A and n = 30 influenza B).

They collected air samples at distances of 1, 3, and 6 feet from the patients' heads using 6-stage Anderson air samplers in 20-minute runs for reverse transcriptase polymerase chain reaction (RT-PCR) testing. During the sampling, patients were not using face masks or oxygen masks, and no aerosol-generating procedures were performed. All test rooms had turbulent air flow through filters of 6 exchanges/hour.

The researchers detected aerosolized influenza in samples from 26 (43%) of the influenza patients (13 inpatients and 13 in the ED). Rapid testing matched the results of RT-PCR testing of nasopharyngeal swabs with air samples.

Higher Nasopharyngeal Viral Loads in Emitters

They found no significant differences in the demographic characteristics of those who emitted particles and those who did not. However, emitters had higher nasopharyngeal viral loads, and their increased virus release by coughing or sneezing was associated with more severe illness, and for the ED patients, an increased degree of interference with their daily lives. Five of the 26 emitters had particularly high levels of RNA copies detected in their air samples compared with the other 21 emitter patients.

"There is a decrease [in the number of particles] from close to the patient to a distance of 6 feet...and it is interesting to note that the small particles — less than 4.7 [µm] — actually made up the majority of particles that we could collect carrying influenza in this setting," Dr. Bischoff reported. At a distance of 1 foot, more than three quarters of the particles were of this size, whereas at 3 feet, the samples contained only about 40% small particles. At 6 feet, the total number of particles was greatly diminished, but almost all of them were small.

Dr. Bischoff noted that a limitation of the study was that it measured only exposure to influenza aerosols and did not evaluate transmission of disease. It also may have overenrolled sicker patients with high amounts of influenza.

He concluded that 43% of influenza-infected patients released virus into the room, that these particles were detected up to 6 feet away, and that virus was predominantly contained in particles smaller than 4.7 µm. Dispersal of virus-carrying particles was associated with high nasopharyngeal viral load, more severe illness, and a greater impact on daily life.

Session moderator James Crowe Jr, MD, professor of pediatrics, pathology, and microbiology and immunology and director of the Vanderbilt Vaccine Center at Vanderbilt University in Nashville, Tennessee, told Medscape Medical News that the study suggests the need to consider whether healthcare workers who are not directly contaminated by large-particle fomites are at risk. "I certainly emphasize that anybody going into these rooms really ought to be immunized, and I guess it raises the question whether we ought to add masking with small-particle masks for those who enter a zone as close as 6 feet," he said.

He noted the long history of using contact precautions for respiratory viruses such as influenza involving "gloves, gowns, and masks if you're going to be directly coughed on." But traditionally, people have not used small-particle aerosol isolation such as one would use for measles or tuberculosis because it is thought that influenza does not travel very far from the patient in such particles.

"This study was looking at 1, 3, and 6 feet away and collecting small particles. And the implication of the study is that small-particle aerosols do at least project virus into the environment, but they don't do it very far," Dr. Crowe noted. "You still might be exposed to small-particle aerosols...within a short distance."

He said that typical paper surgical masks are not designed to stop particles of this size, but N95 fitted respirators should.

Dr. Bischoff has disclosed no relevant financial relationships. Dr. Crowe had no involvement in the study and has disclosed no relevant financial relationships.

IDWeek 2012. Abstract 97. Presented October 18, 2012.