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Study of Flu Transmission Provides More Evidence That Respiratory Viruses Spread via Air

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Influenza Virus

A new publication in PLoS Pathogens by an international team of investigators shows that flu virus might spread via air rather than through contact and large droplet spray, as commonly thought. A large part of the evidence about how respiratory illnesses like COVID-19 are transmitted comes from research on influenza. These new data provide more support for the recommendation by scientists that we include precautions against airborne spread of Covid-19 to have an impact on slowing infection rates.

The study was funded by the US Centers for Disease Control and Prevention to help inform ways to prevent influenza infections, which are responsible for tens of thousands of deaths annually in the United States.  The researchers designed a randomized controlled trial, considered the gold standard for medical proof. They randomized 127 healthy, susceptible volunteers to be either directly infected by influenza via nose drops from a laboratory strain of virus, or to spend four long days in close contact with the directly infected volunteers. Although 42 of the 52 directly infected volunteers had high concentrations of the virus in nasal secretions, they only infected one of the 75 volunteers exposed over four days. The fact that only one person got infected was counter to the researchers’ expectations based on a study conducted in preparation for the randomized controlled trial.

“Our study shows that flu is not transmitted well by contact and large droplet spray, at least not in a controlled experiment,” said Dr. Donald Milton, a senior author of the study who is a professor of applied environmental health in the University of Maryland School of Public Health. “It is quite possible that Covid-19 is not either and that transmission by aerosols, which are sometimes called ‘microdroplets,’ may play a more important role.”

This study’s infection rate was significantly lower than in the earlier smaller study using the same virus to infect volunteers. What was different? Milton says that the main distinction between these studies was that the study participants in the earlier study were in a hotel room with much less ventilation. “Because the main difference between these studies was the ventilation, that indicates that aerosol transmission may be an important mode of influenza virus transmission,” Milton said.

The finding that improved ventilation may limit the spread of influenza has profound implications for control of the spread of COVID-19.

“We’ve been studying the flu for 102 years and still don’t know for sure how it’s transmitted,” said Milton. “The low transmission rate in this study shows how hard it is to nail down how respiratory viruses are transmitted. Given that Covid-19 has more than five times the mortality rate of influenza, we need to heed the data from laboratory and observational studies and exercise appropriate caution with the information we have that suggests that Covid-19, like influenza, may be transmitted by aerosols.”

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