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Mucus protects airborne flu in humid conditions

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June 16, 2018

Seema Lakdawala
Seema S. Lakdawala

For the first time, researchers found that influenza remains highly stable and infectious in aerosols in various levels of humidity, suggesting that mucus and other airway secretions protect the virus when it is expelled, according to study findings published in the Journal of Infectious Diseases.

The results are contrary to the prevailing wisdom that influenza loses infectivity as humidity increases, researchers said.

“I was astonished. The flu virus remained just as infectious at all humidity levels,” Seema S. Lakdawala, PhD, assistant professor in the department of microbiology and molecular genetics at University of Pittsburgh School of Medicine, said in a news release. “The airway secretions were protecting it for at least the length of time it would take a typical home to exchange most of its air.”

Lakdawala and colleagues studied the viability of the 2009 pandemic influenza A(H1NI) virus in suspended aerosols and stationary droplets over a range of relative humidity by creating a custom rotating drum designed to minimize the loss of aerosols.

They noted that extra cellular material (ECM) collected from the apical surface of human bronchial epithelial (HBE) cell cultures at an air liquid interface is “a source of mucosal that may help recapitulate a biologically relevant system to explore virus viability.” They mimicked a physiologically relevant composition of aerosols and droplets by supplementing the suspension medium with HBE ECM. The mixture was sprayed into the rotating drum to mimic mucosal spray.

Seven relative humidity levels were tested, ranging from those found in dry climates or heated indoor environments to those found in very humid rainy periods. In each relative humidity tested, the researchers observed a less than 0.5-log reduction in the amount of infectious H1N1 in viral aerosols aged for 1 hour when present with HBE ECM. There was a very little drop in infectivity in stationary droplets containing HBE ECM.

“Our findings highlight the importance of mimicking real-world conditions when determining the infectivity of emerging viruses,” Lakdawala said. “This has critical implications when public health organizations devise ways to stem the spread of infections, particularly during pandemics.”

According to the researchers, the data “support the notion that the influenza virus may have evolved to exploit host protective barriers to support efficient airborne transmission via creation of a stable microenvironment for the viruses released into the air.” They said the findings “raise a host of questions for future investigations.”

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“Studies are required to identify the contribution of other factors, including temperature and virus strain background, that may also affect viability of influenza virus in aerosols and contribute to the seasonal emergence of influenza virus,” they wrote.

Photo of Linsey Marr
Linsey C. Marr

“The result was surprising because our previous work suggested that the flu virus survived better at low humidity,” Linsey C. Marr, PhD, professor of civil and environmental engineering at Virginia Tech, said in the release. “We thought this might help explain why flu season occurs in the winter, when humidity is low indoors, but now we have to rethink what’s happening with the virus when it’s in droplets and aerosols.” – by Bruce Thiel

Disclosures: The authors report no relevant financial disclosures.

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