This year’s flu season has been particularly severe. As a fast-moving new strain known as subclade K continues to circulate, researchers have released new findings that may help explain how influenza spreads and how people might reduce their risk of infection.

An Unusual Flu Experiment with a Surprising Result

Scientists from the University of Maryland Schools of Public Health and Engineering in College Park and the School of Medicine in Baltimore set out to better understand flu transmission. To do this, they placed college students who were already sick with influenza in a hotel room alongside healthy middle-aged adult volunteers. Despite prolonged close contact, none of the healthy participants developed the flu.

“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.

Studying Airborne Flu Transmission in Real Conditions

Published in PLOS Pathogens, the study is the first controlled clinical trial to closely examine how influenza spreads through the air between people who were naturally infected and people who were not infected. Unlike earlier research that relied on laboratory-infected participants, this trial focused on real-world cases. Milton and his colleague, Dr. Jianyu Lai, analyzed 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 nasal passages, Lai noted that they coughed very little. This limited how much virus entered the air around them.

Ventilation also played an important 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.

Another factor may have been age. Lai explained that middle-aged adults tend to be less susceptible to influenza than younger adults, which may have further reduced the chances of infection.

What the Findings Mean for Flu Prevention

Many experts believe that airborne spread plays a major role in the transmission of influenza. However, Milton emphasized that changing international infection-control guidelines requires strong evidence from randomized clinical trials like this one. The research team is continuing to investigate how much flu spreads through inhalation and under what conditions airborne transmission occurs.

The absence of infections in this study provides useful insight into how people might better protect themselves during flu season.

“Being up close, face-to-face with other people indoors where the air isn’t moving much seems to be the most risky thing – and it’s something we all tend to do a lot. Our results suggest that portable air purifiers that stir up the air as well as clean it could be a big help. But if you are really close and someone is coughing, the best way to stay safe is to wear a mask, especially the N95,” said Milton.

Inside the Hotel-Based Flu Study

The experiment took place on a quarantined floor of a hotel in the Baltimore area. It involved five participants with confirmed influenza symptoms and 11 healthy volunteers, studied across two separate groups in 2023 and 2024. A similar quarantine approach was used in an earlier study, and exhaled breath testing had been featured in several previous influenza studies led by Milton and his colleagues.

During the two-week study period, participants lived together on the isolated hotel floor and took part in daily activities designed to reflect real-life social interactions. These included casual conversations and group movement activities like yoga, stretching, and dancing. Participants with influenza also handled shared items such as a pen, tablet computer, and microphone, which were then circulated among the group.

Extensive Monitoring and Air Sampling

Throughout the study, researchers collected detailed data, including daily symptom reports, nasal swabs, saliva samples, and blood tests to measure antibody responses. They also tracked viral exposure in the air surrounding participants as well as in the general room environment. Each day, exhaled breath samples were collected using the Gesundheit II machine, invented by Milton and colleagues at Harvard T.H. Chan School of Public Health.

Why Controlling Flu Spread Matters

Developing better strategies to limit flu outbreaks remains a major public health priority, according to Milton. Influenza causes widespread illness in the United States and around the world. Each year, up to 1 billion people globally contract seasonal influenza. In the United States alone, the current season has already produced at least 7.5 million cases, resulting in 81,000 hospitalizations and more than 3,000 deaths.