A room full of flu patients and no one got sick
In a striking real-world experiment, flu patients spent days indoors with healthy volunteers, but the virus never spread. Researchers found that limited coughing and well-mixed indoor air kept virus levels low, even with close contact. Age may have helped too, since middle-aged adults are less likely to catch the flu than younger people. The results highlight ventilation, air movement, and masks as key defenses against infection.
This year's flu season has been especially harsh, driven in part by the rapid spread of a new variant known as subclade K. As cases rise, a newly released study offers surprising insight into how influenza spreads and how people may better protect themselves from getting sick.
To better understand how flu moves from person to person, researchers from the University of Maryland Schools of Public Health and Engineering in College Park and the School of Medicine in Baltimore designed an unusual experiment. College students who were already infected with influenza were placed in a hotel room with healthy middle-aged adult volunteers. Despite close contact, none of the healthy participants became infected.
"At this time of year, it seems like everyone is catching the flu virus. And yet our study showed no transmission -- what does this say about how flu spreads and how to stop outbreaks?" said Dr. Donald Milton, professor at SPH's Department of Global, Environmental and Occupational Health and a global infectious disease aerobiology expert who was among the first to identify how to stop the spread of COVID-19.
Why the Flu Did Not Spread
The study, published on January 7 in PLOS Pathogens, represents the first controlled clinical trial to closely examine airborne flu transmission between people who were naturally infected, rather than intentionally infected in a laboratory, and people who were not infected. Milton and his colleague Dr. Jianyu Lai explored several reasons why none of the volunteers became ill.
"Our data suggests key things that increase the likelihood of flu transmission -- coughing is a major one," said Dr. Jianyu Lai, post-doctoral research scientist, who led data analysis and report writing for the team.
Although the infected students carried high levels of virus in their noses, Lai explained that they rarely coughed. As a result, only small amounts of virus were released into the air.
Ventilation also played a key role. "The other important factor is ventilation and air movement. The air in our study room was continually mixed rapidly by a heater and dehumidifier and so the small amounts of virus in the air were diluted," Lai said.
Age may have been another protective factor. According to Lai, middle-aged adults tend to be less vulnerable to influenza than younger adults, which likely contributed to the absence of infections.
What This Means for Flu Prevention
Many scientists believe that airborne transmission is a major driver of flu spread. However, Milton emphasized that changes to global infection-control guidelines require strong evidence from randomized clinical trials like this one. The research team is continuing its work to better understand how flu spreads through inhalation and under what conditions that transmission is most likely.
The lack of transmission observed in this study provides valuable clues about how people can reduce their risk during flu season.