Jan 23, 2026 12:00:00

Why did no new infections appear in an experiment in which influenza patients and healthy people spent several days together?

The influenza virus is known to be highly contagious, and can be transmitted through aerosols released when an infected person coughs or sneezes, as well as through contaminated doorknobs, smartphones, etc. Experiments have been conducted in which infected and healthy people were placed in the same space and allowed to spend several days together, but surprisingly, no spread of infection to the healthy people was observed.

Evaluating modes of influenza transmission (EMIT-2): Insights from lack of transmission in a controlled transmission trial with naturally infected donors | PLOS Pathogens

https://journals.plos.org/plospathogens/article?id=10.1371/journal.ppat.1013153

Why a flu transmission experiment didn't spread the flu
https://theconversation.com/why-a-flu-transmission-experiment-didnt-spread-the-flu-273859

There are various factors that affect the spread of influenza virus infection, such as the amount of virus excreted by an infected person, the temperature and humidity of the room, and the distance from the infected person. Therefore, a research team from the University of Maryland School of Medicine in the United States conducted an experiment using people who had naturally contracted influenza.

The experiment involved participants infected with influenza virus (donors) and healthy subjects (recipients) who were mixed together in a hotel room. Two versions of the experiment were conducted: one with one donor and eight recipients, and the other with four donors and three recipients. Donors were aged 20-22, and recipients were aged 25-45.

The hotel rooms were kept at temperatures (22-25°C) and humidity levels (20-45%) considered conducive to influenza transmission, and ventilation was intentionally reduced by closing all major air passages, including windows, doors, and fan leaks.

The subjects spent three to seven days together in the same room, playing card games in close proximity, participating in dance and yoga classes, and sharing items such as markers, microphones, and tablets.

The researchers monitored the transmission of the virus by measuring the concentration of virus in the donors' breath, saliva, and oral swabs. They also tested for virus particles in shared objects and the air, and recorded common flu symptoms such as coughing, sneezing, and headaches.

The results showed that infectious influenza virus was detected in several samples taken from the donors, but no recipients tested positive for the virus. Although several recipients reported symptoms such as mild headaches, none of the patients showed any evidence of influenza virus infection.

The research team cited three reasons why the infection did not spread despite the close contact between the donor and recipient: low viral shedding from the donor, partial immunity in the recipient, and good air circulation in the room.

While influenza epidemics are generally thought

to be more prevalent in children , the study participants were all adults, and the strain and age of the donors, as well as the fact that they rarely exhibited symptoms such as coughing or sneezing, may have contributed to the low levels of flu virus shed.

Additionally, the adult recipients had survived multiple flu seasons before, which may have given them some immunity, and some had received flu vaccines before, including one who had received the flu vaccine during the study season.

The temperature and humidity in the room were set to conditions favorable for influenza virus infection, but the strong air circulation from the fans in the room may have spread the virus-laden air. This means that the virus was not retained around the donor, but was instead dispersed, potentially reducing the amount of virus inhaled by the recipient.

'The findings from this study suggest that coughing and sneezing are the main factors in spreading influenza viruses, and that super-spreaders who release large amounts of virus play a particularly important role. The immune status of people exposed to influenza viruses and the airflow in indoor spaces are also likely important factors.'

Of course, influenza is a dangerous infectious disease that infects hundreds of millions of people worldwide every year, so we should never let our guard down. Because there is a particularly high risk of aerosol transmission through coughing and sneezing, patients with these symptoms should be isolated as much as possible and wear tightly fitting masks to prevent the virus from spreading through the air. Also, adequate ventilation and air circulation are essential in poorly ventilated spaces.