This year's flu season has been marked by the rapid spread of a new variant, subclade K. A recent study published in PLOS Pathogens provides insights into how influenza is transmitted and how outbreaks might be prevented.
Researchers from the University of Maryland Schools of Public Health and Engineering, along with the School of Medicine in Baltimore, conducted a controlled clinical trial to investigate airborne transmission of the flu. In their experiment, college students who were already sick with influenza were placed in a hotel room with healthy middle-aged adult volunteers. Despite close contact over two weeks, none of the healthy participants contracted the virus.
"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.
The study is notable for being the first clinical trial to examine natural airborne transmission between infected and uninfected individuals in a controlled setting. Dr. Jianyu Lai, post-doctoral research scientist and lead analyst for the team, pointed out that coughing plays a significant role in spreading influenza: "Our data suggests key things that increase the likelihood of flu transmission – coughing is a major one."
Lai explained that while infected students had high levels of virus in their noses, they rarely coughed during the study period, resulting in minimal viral particles being released into the air. He also noted that middle-aged adults are generally less susceptible to influenza than younger people.
Dr. Milton emphasized that updating international infection-control guidelines requires evidence from randomized clinical trials like this one. The team's ongoing research aims to further clarify how much airborne inhalation contributes to flu transmission.
Milton offered practical advice based on their findings: "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."
The experiment took place on an isolated floor of a Baltimore-area hotel using five confirmed influenza cases and 11 healthy volunteers across two cohorts during 2023 and 2024. Participants engaged in daily activities such as conversation exercises and physical activities like yoga or dancing; they also shared objects handled by those infected.
Throughout the study, researchers monitored symptoms daily and collected nasal swabs, saliva samples, and blood for antibody testing. Airborne viral exposure was measured both in participants' breathing zones and ambient room air using specialized equipment developed by Milton's team at Harvard T.H. Chan School of Public Health.
Dr. Milton stressed that controlling flu outbreaks remains a public health priority given its global impact—seasonal influenza affects up to 1 billion people worldwide each year; so far this season there have been at least 7.5 million cases in the United States alone with 81,000 hospitalizations and more than 3,000 deaths.
Contributors included members from UMD's Public Health Aerobiology Lab as well as collaborators from mechanical engineering departments at UMD, University of Maryland School of Medicine, Icahn School of Medicine at Mount Sinai (New York), University of Hong Kong, and University of Michigan Ann Arbor.
