Study: Airborne flu viruses may play big role in transmission

Feb 1, 2013 (CIDRAP News) – Patients with flu in hospital settings often produce small virus-containing particles during routine care that may travel up to 6 feet, casting doubt on whether current infection control measures are enough to protect healthcare workers, according to a new study.

Researchers also found that some of the flu patients were "super spreaders" who may be more likely to spread the disease than others. The findings, by a team from Wake Forest School of Medicine in North Carolina, appeared this week in the Journal of Infectious Diseases.

The role of aerosols in flu transmission has been murky and controversial. The topic became a hot-button issue during the 2009 H1N1 influenza pandemic when federal officials, professional groups, and hospitals were faced with making science-based recommendations to protect health workers, using often-inconclusive data.

Many recommendations are based on the thought that flu viruses are primarily spread by large-particle droplets that travel relatively short distances, typically less than 3 feet from an infected person. The scientific literature is less clear about the role of small-particle aerosols that can travel longer distances and be inhaled more deeply into the lungs.

The study took place during the 2010-11 flu season in the emergency department (ED) and inpatient care units at Wake Forest Baptist Medical Center. Researchers used a convenience sample of 94 patients ages 2 and older who were admitted to the ED or to inpatient care and were screened for flulike illness. The facility has a mandatory flu vaccination policy for its healthcare workers.

Respiratory samples were taken for rapid testing and for real-time reverse-transcription polymerase chain reaction (rRT-PCR) testing. Researchers also used air samplers to assess the patterns of airborne flu viruses. The devices were placed about 1, 3, and 6 feet from the patients' head level. No aerosol-generating procedures such as bronchoscopy were done during air sampling.

Patients were administered a questionnaire about their disease symptoms and were asked to report their illness severity and its interference with daily activities, using a visual analog scale.

During illness screening, the research team counted and assessed patients' coughs and sneezes.

Sixty-one (65%) of the 94 patients tested positive for influenza, and 26 (43%) of those flu patients released influenza viruses into the air, according to the study.

Five (19%) of the 61 patients—the "super spreaders" or "super emitters"—produced up to 32 times more viruses than others. Researchers found that those patients had high viral loads in their respiratory samples and reported having greater illness severity.

In addition, the investigators found that emitters exceeded the airborne 50% human infectious dose of flu at all sample locations.

When the team analyzed the air samples, they found that 89% of flu viruses were found in small particles (less than 4.7 micrograms) up to 6 feet from the patient's head, with concentrations that decreased with distance. Researchers emphasize that the study only identified virus-laden particles near patients during routine care and didn't gauge actual transmission.

They concluded that health workers could be exposed to infectious doses of flu viruses up to 6 feet from patients, which calls into question current recommendations. Standard procedures focus on droplet precautions requiring surgical masks for routine care and special instructions including N-95 respirator use during aerosol-generating procedures.

From the patients' symptom questionnaires, the authors found that more severe illness and greater impact on daily life were associated with increased flu virus release. Though targeting "super emitters" might offer a more efficient way to control flu transmission, the group wrote, the limited sample size didn't allow them to flag specific symptoms or patient characteristics.

The researchers cautioned that the PCR method doesn't distinguish between viable and nonviable virus, and the study design offers only a snapshot of flu virus emission dynamics. They also noted that the convenience sample excluded asymptomatic infections and could favor more severely ill patients who have higher viral loads.

In an editorial that accompanied the study, Caroline Breese Hall, MD, who was professor of pediatrics and medicine at the University of Rochester School of Medicine and Dentistry in New York, wrote that the controversy over airborne transmission hasn't been about whether such transmission occurs, but rather about the degree of its contribution to the spread of flu. (Hall died on Dec 10 soon after she wrote the editorial, according to a press release yesterday from the Infectious Diseases Society of American [IDSA].)

She wrote that even if airborne spread occurs less commonly than close contact, its importance is potentially great. "The infectious dose is much less, the source more difficult to identify, and preventive infection control procedures are more complex and expensive," Hall said.

So far the role of "super spreaders" is also unclear, but heavy shedders may be the source of a major proportion of influenza virus infections, she said.

Hall wrote that it's difficult to assess whether current research developments are poised to steer any changes in current recommendations for routine care of patients with flulike illness. More advanced personal protective equipment is expensive and may not be feasible when caring for all patients with flulike illness, she wrote.

She also noted that self-reporting of symptoms as a gauge of super-spreader status isn't likely to be sensitive or specific enough for use in patient-care settings.

"A tenable conclusion from these findings is that infection control procedures should be commensurate with the concern generated by the clinical observations of the intensity and severity of the community outbreak," she wrote, adding that the efficacy of infection control programs depends less on specific procedures than on consistent education of health personnel, including vaccination, compliance with recommendations, and awareness of infection-control risks.

"This study not only adds to our understanding of these risks, but helps define the questions that still need answering," Hall wrote.

Bischoff WE, Swett K, Leng I, et al. Exposure to influenza virus aerosols during routine patient care. J Infect Dis 2013 Jan 30 [Abstract]

Hall CB. Influenza virus: here, there, especially air? (Editorial) J Infect Dis 2013 Jan 30 [Extract]

See also:

Jan 31 IDSA press release

Dec 2, 2010, CIDRAP News story "Study: Airborne particles from coughs can carry flu viruses"

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