Sometimes you can’t get away from sick people–but YOU don’t have to get sick

Would you, as a healthy person, volunteer to spend time with a flu patient?  In an experiment, college students who were already infected with influenza were placed in the same room with healthy middle-aged adult volunteers. Despite close contact, none of the healthy participants became infected.  It shows that although the flu is going around, it doesn’t have to infect you. 

Five naturally-infected (not artificially infected) students (called Donors) were placed with 11 healthy volunteers (Recipients).  Participants lived on an isolated hotel floor for two weeks and were brought together in “exposure events” lasting from 111 to 250 minutes.  These events were designed to mimic real-life social interactions, including casual conversations, interactive games like UNO, and physical activities such as yoga, stretching, and dancing. Infected participants also handled items like a pen, tablet computer, and microphone, which were then passed around the group.

Throughout this time, researchers tracked symptoms and collected nasal swabs, saliva samples, and blood samples to monitor infection and antibody development. They also measured viral exposure both in the air participants breathed and in the room itself. (A room full of flu patients and no one got sick)

What prevented the healthy people from getting sick?

• The hotel room had limited ventilation (fresh air) but a high air recirculation rate, serving to mix the air well.  During the exposure events, two fan coil units and two large capacity dehumidifiers with fans ran constantly on high speed.  It has been shown in previous studies that high rates of air mixing reduce the extent of Recipient exposure to concentrated plumes of exhaled breath.

• Some Recipients were assigned “intervention” equipment, such as face shields and hand hygiene.

• Even though the donors had high nasal virus concentrations, they rarely coughed. 

• Admittedly the years 2023-24 when the experiments ran, it was a “mild” flu season and “contagiousness” of the Donors was low.

• The Recipients may have had pre-existing immunities: some had received a flu vaccine and since Recipients were middle-aged adults, they would have had greater cumulative exposure to influenza virus infections and vaccinations than Donors (college-age).

The indoor air temperature was within 22-25 degC (roughly 72-77 degF), and relative humidity ranged between 20–45%; these ranges actually would promote flu transmission. The ventilation rate was approximately 0.25-0.5 air changes per hour, which is fairly low.  Low air change rates have been linked to increased risk in healthcare and public settings.  (Evaluating modes of influenza transmission (EMIT-2): Insights from lack of transmission in a controlled transmission trial with naturally infected donors)

Even if the Recipients did not “get” the flu according to PCR flu tests, they did sporadically report symptoms, with most Recipients reporting more symptoms after exposure to a Donor.  Sounds pretty normal to us!

So what can we learn from this trial?  If you have to care for sick individuals, or be in their proximity, increasing ventilation (fresh air) is preferred.  However, if you can’t increase ventilation, increasing air circulation (mixing) is a good alternative.  This is because it breaks up concentrated virus plumes coming from infected individuals.  Ceiling, floor or table fans, air purifiers, air conditioners, or exhaust fans all help in this situation.  The trial was designed to keep the atmosphere “hospitable” for transmission, when in reality you would want to do the opposite.  Therefore, keeping the air a bit more humid (40-60%) reduces virus survival and infectivity.  In summary, to keep the flu at bay, keep that air moving and not too dry!  We would also suggest “interventions” such as good-fitting masks if there’s a lot of coughing going on.

Photo by Towfiqu barbhuiya on Unsplash