If a healthy person enters a room full of flu patients, will they catch the flu? Most people would think, "Yes." In fact, an experiment was conducted where healthy individuals spent several days in the same space as flu-infected individuals. What were the results? It was reported that not a single person caught the flu... What factors were at play?
A joint research team from the University of Maryland's School of Public Health, School of Engineering, and School of Medicine conducted the first controlled clinical trial to precisely observe airborne transmission among naturally infected flu patients and non-infected individuals, and published the results in the international journal ⟪PLOS Pathogens⟫ on January 7.
According to reports from scientific media such as Science Daily, the research team utilized a quarantined floor of a hotel near Baltimore as the experimental space over the years 2023 and 2024. The experiment was conducted in two-week isolation cohorts, where healthy volunteers lived in a state cut off from the outside world for about 14 days from the time of admission. During this isolation period, some times were designed for naturally infected flu patients to participate in 'exposure situations.'
There were a total of 5 naturally infected flu patients, all of whom had confirmed influenza infections prior to participating in the experiment. The healthy participants who came into contact with them totaled 11, with an average age in the mid-30s. To create conditions similar to real life rather than just 'brief contact,' participants were encouraged to converse and engage in activities together in the same space.
A total of 23 exposure situations were conducted, each lasting about 2 to 4 hours. The total exposure time amounted to approximately 82 hours. During this period, participants engaged in free conversations, played card games, and participated in light physical activities such as yoga, stretching, and dancing. Situations involving the sequential use of shared items like pens, tablets, and microphones were also included.
The ventilation conditions of the space where the experiment was conducted were intentionally limited. Although the ventilation rate due to external air inflow was low, heaters, dehumidifiers, and fans were continuously operated to ensure rapid mixing of indoor air. The research team created an environment to prevent viruses from remaining in high concentrations at specific points in the air.
During the experiment, the research team monitored participants' symptoms daily and repeatedly conducted nasal swabs, saliva, and blood tests. Determining infection status was not based solely on the occurrence of simple symptoms but included molecular diagnostics (PCR) and serological tests.
Living Continuously with Flu Patients... No One Caught the Flu, Why?
Analysis revealed that none of the healthy participants showed flu symptoms, and no influenza virus was detected in PCR tests. Blood tests also did not show any signs of antibody increases indicating new infections.
The research team proposed several factors for the absence of flu transmission. First, the frequency of coughing among the infected individuals was very low. While high levels of the virus were detected in the nasal passages, it was analyzed that the actual amount of virus expelled into the air was likely limited. Additionally, as the indoor air was continuously mixed, the likelihood of the virus from the infected individuals being transmitted in high concentrations to specific individuals may have decreased.
Professor Milton, who led the research, stated, "Portable air purifiers that both purify and circulate air can help reduce the risk of indoor infections," and added, "However, if someone is coughing very close to you, the most effective protective measure is wearing an N95 mask."
It was also suggested that the age and immune status of the healthy participants may have influenced the results. Existing research indicates that middle-aged individuals are relatively less susceptible to influenza compared to younger adults. The research team interpreted that these factors acted in combination, resulting in no infections despite prolonged close indoor exposure.
This Does Not Mean Flu Is Not Easily Transmitted... Improving Air Circulation and Ventilation Is Important
However, the research team clarified that these results do not mean that flu is not easily transmitted. Rather, it emphasizes that flu transmission can be significantly influenced by behaviors such as coughing, indoor air flow, and ventilation conditions, rather than just distance or cohabitation. It particularly suggests that situations where air is stagnant indoors and faces are close together while coughing may be the most dangerous combination.
Accordingly, the research team emphasized that improving air circulation and ventilation in indoor spaces can help reduce infection risks, and that wearing high-performance masks remains the most effective protective measure in close contact situations. Given that flu causes a significant disease burden worldwide every year, the research team stated that further studies are needed to verify flu airborne transmission under various conditions.
Professor Milton remarked, "During flu seasons, it feels like everyone around is getting infected, but this study observed no transmission at all," and added, "This result makes us rethink how flu spreads and how to prevent outbreaks."
