In a unique hotel quarantine experiment, University of Maryland researchers found that flu did not spread from sick students to healthy adults. Their findings point to practical ways to cut risk this season, from better ventilation to masks.
As a harsh flu season sweeps the United States, a new study from the University of Maryland is offering rare good news: under certain conditions, even close contact with people who have the flu may not lead to infection.
In a controlled hotel experiment, researchers brought together college students who were already sick with influenza and healthy, middle-aged adult volunteers. Despite living, talking, exercising and sharing objects together on a quarantined hotel floor, none of the healthy participants caught 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?” Donald Milton, a professor in the School of Public Health’s Department of Global, Environmental and Occupational Health and a global infectious disease aerobiology expert, said in a news release.
The study, published today in PLOS Pathogens, is the first clinical trial in a controlled environment to focus on how flu spreads through the air from people who are naturally infected, rather than from volunteers deliberately infected in a lab. That distinction matters because real-world infections can behave differently from lab-induced ones.
Milton and his colleagues wanted to pin down how much airborne transmission contributes to flu spread and what conditions make it more or less likely. To do that, they turned a floor of a Baltimore-area hotel into a temporary research unit in 2023 and 2024.
On the quarantined floor, five people with confirmed influenza and symptoms lived alongside 11 healthy volunteers in two cohorts. For two weeks, participants took part in daily activities designed to mimic everyday life: conversational ice-breakers, group stretching, yoga, dancing and other shared routines. Infected participants handled common items such as a pen, tablet computer and microphone before passing them around to the entire group, creating opportunities for both airborne and surface transmission.
The researchers closely tracked what happened. They monitored symptoms, collected daily nasal swabs and saliva samples, and drew blood to look for antibodies that would indicate new infections. They also measured how much virus was present in the air around the volunteers’ breathing zones and in the activity room itself.
Each day, infected participants breathed into a specialized device called the Gesundheit II, which captures and measures virus in exhaled breath. Milton helped invent the machine while at the Harvard T.H. Chan School of Public Health, and it has been used in several earlier studies of influenza transmission.
Despite all that close contact, no new flu infections appeared among the healthy adults.
“Our data suggests key things that increase the likelihood of flu transmission – coughing is a major one,” added Jianyu Lai, a post-doctoral research scientist, who led the team’s data analysis and report writing.
The sick students carried a lot of virus in their noses, Lai found, but they were not coughing much. Without frequent, forceful coughs to blast virus-laden droplets and aerosols into the air, far less virus reached the breathing space of others.
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 added.
That constant mixing and dilution likely kept airborne virus levels below the threshold needed to infect the volunteers. Lai noted that middle-aged adults also tend to be less susceptible to influenza than younger adults, which may have further reduced the odds of transmission in this particular group.
Most scientists already consider airborne transmission a major driver of flu spread, alongside larger respiratory droplets and contaminated surfaces. But public health agencies often look for evidence from randomized clinical trials before updating infection-control guidelines. Milton’s team designed this trial, known as EMIT-2, to help provide that kind of evidence.
The lack of transmission in this setting does not mean flu is hard to catch. Seasonal influenza still causes a heavy burden of disease worldwide, with up to 1 billion infections globally each year. In the United States, this season alone has already seen millions of cases, tens of thousands of hospitalizations and thousands of deaths, according to federal estimates.
Instead, the findings highlight specific conditions that appear to make flu spread more or less likely — and point to practical steps people can take to protect themselves.
The riskiest situations are familiar ones: crowded, still-air indoor spaces where people are close together and someone is coughing.
“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,” Milton added. “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.”
Those recommendations align with broader lessons from the COVID-19 pandemic, when improving ventilation, using high-quality masks and avoiding crowded indoor gatherings helped reduce transmission of another respiratory virus.
The University of Maryland team emphasized that their work is ongoing. Future phases of the EMIT-2 project are designed to better quantify how much virus people inhale in different conditions and how that relates to infection risk. The researchers also aim to clarify how airborne particles of different sizes contribute to transmission.
The study drew on expertise from across the university and beyond. Members of UMD’s interdisciplinary Public Health Aerobiology Lab, including Kristen Coleman, Yi Esparza, Filbert Hong, Isabel Sierra Maldonado, Kathleen McPhaul and S.H. Sheldon Tai, contributed to the work. Collaborators from the University of Maryland’s Department of Mechanical Engineering, the University of Maryland School of Medicine, the Icahn School of Medicine at Mount Sinai, the University of Hong Kong and the University of Michigan, Ann Arbor, also played roles.
For now, the hotel experiment offers a hopeful message: flu transmission is not inevitable, even when people share space with someone who is sick. With better ventilation, attention to coughing and strategic use of masks and air purifiers, communities may be able to blunt the impact of seasonal influenza — not just this year, but in many winters to come.