A room full of flu patients and no one got sick

To better understand how influenza is transmitted from person to person, researchers from the schools of public health and engineering at the University of Maryland in College Park and the Baltimore College of Medicine designed an unusual experiment. College students who had already had the flu 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, everyone seems to be infected with the influenza virus. However, our study showed no transmission – what does this say about how influenza spreads and how to stop outbreaks?” said Dr. Donald Milton, a professor in the Department of Global, Environmental and Occupational Health at SPH and a world expert in the aerobiology of infectious diseases who was among the first to identify how to stop the spread of COVID-19.

Why didn’t the flu spread?

The study was published on January 7 PLOS pathogensIt represents the first controlled clinical trial to closely examine airborne influenza transmission between people who became infected naturally, rather than intentionally in a laboratory, and people who were not infected. Milton and his colleague Dr. Jianyu Lai explored several reasons why none of the volunteers developed the disease.

“Our data point to key things that increase the likelihood of influenza transmission, and coughing is one of the main reasons,” said Dr. Jianyu Lai, a postdoctoral research scientist who led the team’s data analysis and report writing.

Although infected students carry high levels of the virus in their noses, Lai explained that they rarely cough. As a result, only small amounts of the virus were released into the air.

Ventilation also played a major role. “Another important factor is ventilation and air movement. The air in the classroom was constantly mixed rapidly by a heater and dehumidifier, so small amounts of the virus in the air were diluted,” Lai said.

Age may have been another protective factor. According to Lee, middle-aged adults tend to be less susceptible to influenza than younger adults, which likely contributed to the absence of infection.

What does this mean for flu prevention

Many scientists believe that airborne transmission is the main driver of the spread of influenza. However, Milton stressed that changes in 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 influenza is spread by inhalation and under what conditions the disease is most likely to be transmitted.

The lack of transmission observed in this study provides valuable clues on how to reduce the risk of influenza infection during the influenza season.

“Being in close proximity face-to-face with other people indoors where the air isn’t moving much seems to be the riskiest thing — and it’s something we all tend to do a lot of. Our results suggest that portable air purifiers that move and clean the air can be a big help. But if you’re really close and someone is coughing, the best way to stay safe is to wear a mask, especially an N95.”

Inside the influenza experience

The research was conducted on the isolated floor of a Baltimore-area hotel and included five participants with confirmed flu symptoms and 11 healthy volunteers. The study was conducted in two groups during the years 2023 and 2024. A similar quarantine design has been used in previous research, along with a specialized exhaled breathing test developed by Milton and colleagues.

Participants lived on an isolated hotel floor for two weeks and followed daily routines designed to mimic real-life social interactions. These included informal conversations and physical activities such as yoga, stretching and dancing. Affected participants also handled shared items such as a pen, tablet, and microphone, which were then passed around the group.

The researchers closely tracked symptoms and collected daily nasal swabs, saliva samples and blood samples to monitor infection and the development of antibodies. They measured viral exposure in the air breathed by the participants and in the room itself. Exhaled samples were collected daily using the Gesundheit II machine, invented by Milton and colleagues at the Harvard T. H. Chan School of Public Health.

Why is influenza research still important?

Finding better ways to reduce influenza outbreaks remains a key public health priority, according to Melton. Influenza continues to place a heavy burden on health systems around the world. Every year, up to one billion people worldwide become infected with seasonal influenza. In the United States alone this season, there have already been at least 7.5 million cases, resulting in 81,000 people being hospitalized and more than 3,000 deaths.

The study included contributions from researchers at the University of Maryland’s Interdisciplinary Public Health Aerobiology Laboratory, including Christine Coleman, Yi Esparza, Filbert Hong, Isabel Sierra Maldonado, Kathleen McFall, and S. H. Sheldon Tai, as well as collaborators from the University of Maryland Department of Mechanical Engineering, the University of Maryland School of Medicine, the Icahn School of Medicine at Mount Sinai in New York, the University of Hong Kong, and the University of Michigan, Ann Arbor.

Funding for the research came from an NIAID U19 Cooperative Agreement grant (5U19AI162130), the Baltimore Institute for Clinical and Translational Research (ICTR) at the University of Maryland, the University of Maryland Strategic Partnership: MPowering the State (MPower), and gifts from the Influenza Laboratory and the Balvi Filantropic Fund.