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In a new study researchers had five people infected with the flu mingle with 11 people who did not have the virus in a hotel room with limited ventilation, but with a high rate of air circulation.
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In the end, none of the healthy participants got sick, which may be due to the age of the participants (and natural immunity to the flu), among other factors.
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Doctors say this suggests that keeping air circulating and wearing an N95 mask can be helpful in preventing the spread of the flu.
Common advice on reducing your risk of flu is to avoid being around people who are obviously sick. But scientists recently did the opposite, putting people who had the flu in the same room as healthy participants, as part of a small new study. (All participants provided written informed consent before enrolling in the study, and retained the right to withdraw at any time.)
Shockingly, no one was infected.
“Our goal in this study was not to prevent the flu,” says infectious disease aerobiology expert Donald Milton, MD, study co-author and professor in the Department of Global, Environmental and Occupational Health at the University of Maryland School of Public Health. “We’ve been trying to understand how transmission happens so we can design better ways to prevent it from happening.”
Milton points out that about eight percent of people get the flu during a normal respiratory virus season, making it a fairly infectious virus.
So…why didn’t healthy people get the flu, even when they were in the same room as those who were infected? The findings offer some important tips for flu prevention, whether someone in your household has the virus or you want to play it safe.
Meet the experts: Amesh A. Adalja, MD, is an infectious disease expert and senior scholar at the Johns Hopkins Center for Health Security. Thomas Russo, MD, is a professor and chief of infectious diseases at the University of Buffalo in New York. Infectious disease aerobiology expert Donald Milton, MD, is the co-author of the study and a professor in the Department of Global, Environmental and Occupational Health at the University of Maryland School of Public Health.
What did the study find?
For the PLOS Pathogens study, researchers had five people infected with the flu mix with 11 people who did not have the virus in a hotel room with limited ventilation, but a high rate of air circulation.
Study participants lived on an isolated floor of a hotel for two weeks and performed daily routines that were designed to mimic real-life social interactions, such as casual conversations, yoga, stretching and dancing. The people who were infected touched things like a pen, tablet, and microphone, which were then passed around the group.
During this time, the researchers followed the participants’ symptoms and collected daily data, such as nasal swabs, saliva and blood samples. They also measured the levels of the virus in the air and in the participants’ breath.
Ultimately, none of the healthy participants got sick.
Why hasn’t everyone been infected?
There are probably a few reasons for this. “Many people find it shocking to realize that every time scientists have tried to put people in a room to see if infected people infect uninfected people with the flu, it hasn’t worked,” says Dr. Milton. “It’s strange.”
His team thought this was because previous studies used laboratory viruses that circulated years ago (and to which people had built up immunity), so they decided to use people who were naturally infected with current strains of the virus.
But Dr. Milton thinks one reason why the healthy participants didn’t get sick is that they were middle-aged adults who had years of exposure to the flu. As a result, they may have more natural immunity than younger people, he says.
The study was also conducted during a milder flu year (2023-2024), aka not this year, points out Dr. Milton. While most of the study participants had high levels of the flu virus in their noses, they did not cough. “In previous years, we saw that people who don’t cough don’t really spread the virus,” he says. “Influenza cases where people are coughing are the most contagious.”
Dr Milton says the ventilation was “intentionally quite poor”, with researchers sealing the doors and doing what they could to limit ventilation. But the air was also moving a lot around the room. “We wanted to have a well-mixed environment, thinking it would get everyone exposed,” he says. “It turned out that it worked the other way around. There was enough air in the room that when you had people who weren’t coughing much, everyone was exposed to a small amount of virus, which wasn’t enough.”
What does it take to stay fit?
Doctors say there are a few things you can do with this information, whether you’re out in public or if someone in your home is sick. Limiting coughing—using cough suppressants or having the sick person wear a mask around others—can be helpful, according to Amesh A. Adalja, MD, an infectious disease expert and senior scholar at the Johns Hopkins Center for Health Security.
“Coughing is a major mechanism by which viral particles spread through the air,” he explains. “Reducing coughing, reduces that opportunity to spread respiratory viruses.”
Good airflow is also helpful, says Thomas Russo, MD, professor and chief of infectious diseases at the University of Buffalo in New York. “Usually it is difficult to open the windows during the winter because the entry of cold air inside is quite poor,” he says. “But it’s a reasonable idea to use an air filter at home.” Turning on the ceiling fans can also help, Dr. Milton says. Dr. Adalja agrees. “The better the airflow, the less likely viral particles in the air will land on another individual,” he says.
But one of the best ways to protect yourself is by wearing an N95 mask—especially if you’re around people who are coughing, says Dr. Milton. “At the end of the day, all these layers of protection work—that’s what it shows,” he says.
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