An experiment shows that the conditions that allow influenza viruses to spread are more nuanced and complex than previously believed.
Not out of sadism but for the sake of science, a group of epidemiologists from the University of Maryland (United States) asked some people to share a hotel room for a few hours with other participants suffering from the flu. The intention was to understand who, among the healthy people, would remain infected and why. In the end, however, no one got sick, and the study allowed us to understand a little better about the transmission mechanisms of influenza viruses.
Being close to an infected person is not enough to make you sick
Influenza viruses spread through the air, through thicker or thinner droplets (droplets and aerosols) emitted when we cough, sneeze or even just when we speak, and through contact with surfaces touched by those who have been infected (such as door handles, table surfaces or taps).
The covid pandemic has made simple measures to reduce the transmission of viruses indoors automatic for us, such as sneezing into your elbow, washing your hands well, wearing a mask or working from home if you are unwell. However, as useful as it is to take advantage of these preventive measures, it must be said that being in the same room with a person suffering from flu is not enough to become infected ourselves. The new study, published on PLOS Pathogensis proof of this.
Why no contagion?
American scientists forced (with prior consent!) volunteers – the “donors” – who had contracted the flu naturally, without inoculations in the laboratory, and other healthy participants, the “recipients”, to coexist for a few hours and for a period varying from 3 to 7 days.
In the confined environment, with sealed doors and windows, the small group played cards, danced or practiced yoga, shared markers and tablets passing them from hand to hand. In a first part of the experiment, a single infected donor shared a room with 8 recipients. In the other, four donors shared a room with three recipients. In all cases, the affected people were 20 to 22 years old, the recipients were 25 to 45 years old.
Although the room’s temperature (from 22 to 25°C) and humidity (from 20% to 45%) conditions were specifically designed to favor viral transmission, none of the recipients contracted the flu. The researchers made sure of this by carefully monitoring symptoms such as sneezing, coughing, nose blowing, sampling saliva and analyzing viral particles exhaled by donors or left on surfaces. What had protected the healthy?
Because viral transmission was unsuccessful
The passage of influenza viruses between donors and recipients was blocked by a limited release of the virus by those who were ill (either due to the type of strain contracted or due to the limited number of coughs and sneezes observed in the study), by efficient immune defenses by the recipients, already infected in the past or vaccinated, and by the internal ventilation system of the room.
Even though fresh air exchanges were blocked, internal air recirculation disturbed and diluted the virus-laden clouds exhaled by donors, reducing the amount of virus inhaled by recipients.
The experiment, therefore, confirmed that coughing and sneezing are the main spreaders of influenza viruses, and that previous immunity and good ventilation of closed environments can really make a difference.
