This year, the flu season has been particularly severe, largely due to the rapid spread of a new virus variant known as subclade K. Against the backdrop of rising infection rates, a recently published study sheds light on how influenza is transmitted and highlights effective ways to protect against infection.

An Unusual Experiment: How the Flu Spreads

To better understand the mechanisms of flu transmission from person to person, researchers at the University of Maryland conducted a unique experiment. In this study, college students already infected with the flu were housed in a hotel room together with healthy adult volunteers of middle age. Despite close contact, none of the healthy participants became infected. This surprising result led the researchers to reconsider the actual routes of virus transmission and the possibilities for preventing outbreaks.

Key Factors in Virus Transmission

The study, published on January 7 in the journal PLOS Pathogens, was the first controlled clinical trial to closely examine airborne flu transmission between people naturally infected with the virus, rather than those infected in laboratory settings. The scientists explored several reasons why none of the volunteers got sick. The data showed that one of the main factors in flu transmission is coughing. Although the infected students had high viral loads in their noses, they rarely coughed, so only a small amount of the virus entered the air.

Ventilation also played a crucial role: the air in the room was constantly circulated by a heater and dehumidifier, which quickly diluted even small amounts of virus. Another protective factor may have been age—middle-aged adults are less susceptible to the flu than younger people, which could have contributed to the lack of infections.

The Importance of These Findings for Prevention

Many experts believe that the primary way flu spreads is through airborne droplets. However, to change global infection control guidelines, convincing evidence from randomized clinical trials is needed. The research team continues to work on pinpointing the specific conditions under which flu is most likely to be transmitted through inhalation of the virus.

The absence of virus transmission in this experiment offers valuable insights into how to reduce the risk of infection during flu season. The greatest danger arises during close, face-to-face interactions in poorly ventilated indoor spaces. The results suggest that portable air purifiers, which circulate and clean the air, can be helpful. However, if you are in close proximity to someone who is coughing, the best protection is a mask, especially an N95 respirator.

How the Experiment Was Conducted

The experiment took place on an isolated floor of a hotel near Baltimore and involved five participants with confirmed flu symptoms and 11 healthy volunteers. The study was conducted in two groups during 2023 and 2024. Participants lived on the floor for two weeks, following a schedule that mimicked typical social interactions: conversations, physical exercise, yoga, stretching, and dancing. The infected individuals also used shared items, which were then passed to others.

Researchers closely monitored symptoms, took daily nasal swabs, saliva, and blood samples to track infection and antibody development. They measured the viral load both in the air breathed by participants and in the room itself. Samples of exhaled air were collected daily using a special device called Gesundheit II.

Flu: A Global Challenge

Finding effective ways to contain flu outbreaks remains a major challenge for public health. Every year, influenza places a tremendous burden on healthcare systems worldwide: up to 1 billion people are infected with seasonal flu annually. In the United States alone, this season has already seen at least 7.5 million cases, 81,000 hospitalizations, and more than 3,000 deaths.