Why a flu transmission experiment didn’t spread the flu

A group of volunteers spent days locked in a small hotel room with people actively infected with flu. They played games, shared objects and exercised together in conditions designed to help the virus spread. Yet not a single person caught influenza.

The unexpected finding comes from a well-designed study that set out to answer a basic question: how does flu really spread?

Influenza, the virus responsible for flu, is known to spread through aerosols (microscopic droplets) released when an infected person coughs, sneezes or even breathes normally. It can also pass from person to person via contaminated surfaces such as door handles or phones, known as fomite transmission.

How efficiently the virus spreads depends on several factors, including how much virus an infected person sheds, the temperature and humidity of a room and how close people are to one another.

To tease apart which of these factors matter most, researchers at the University of Maryland in the US ran a real-world transmission experiment using people who had caught flu naturally.

They brought together groups of participants in a hotel room, mixing people with active influenza infections, referred to as donors, with uninfected volunteers, referred to as recipients. The aim was simple: see whether flu would spread under conditions designed to favour transmission.

Despite prolonged close contact over several days, no recipient became infected.

This approach differed from earlier studies in which healthy volunteers were deliberately infected with influenza for research. By using naturally infected “donors”, the researchers hoped to better reflect how flu spreads outside the laboratory.

Two versions of the experiment were carried out. In one, a single donor shared a room with eight recipients. In the other, four donors shared with three recipients. Donors were aged 20 to 22, while recipients were aged between 25 and 45.

The room was kept at temperatures and humidity levels thought to favour influenza transmission, at 22°C to 25°C, and 20% to 45% humidity. Before quarantining the participants, the researchers closed off major uncontrolled air pathways – such as windows, doors and a leak in the fan coil units – to deliberately create low ventilation and poor air quality.

Over three to seven days, participants spent hours together in the confined space. They played card games at close range, took part in dance or yoga classes and passed around shared objects such as markers, microphones or tablet computers.

The researchers monitored transmission by measuring virus levels in exhaled air, saliva and mouth swabs from donors. Shared objects and the room air were also tested for viral particles. Participants recorded symptoms including coughing, sneezing, headaches and other common signs of flu.

Why transmission failed

Several samples from donors confirmed active influenza infection. But none of the recipients tested positive. A few reported mild symptoms such as headaches, but there was no clear evidence of flu infection in any of them.

The researchers suggest three main reasons why transmission may not have occurred: low virus shedding from donors, partial immunity among recipients and the way air circulated in the room.

Children are widely thought to drive the spread of influenza, but this study involved only adults. Adult donors in the experiment released relatively small amounts of virus. This may reflect the strains they were infected with, their age or the fact that they showed few symptoms. Very little coughing or sneezing was observed, which would have limited the amount of virus entering the air.

Recipients may also have been less susceptible. They had all lived through many flu seasons and several had received flu vaccinations in previous years, with one vaccinated in the current season. This prior exposure may have given them some background immunity.

Although temperature and humidity were set to favour transmission, the high level of air recirculation caused by fans may have disrupted clouds of virus-laden air. Instead of lingering around donors, these plumes may have been broken up and diluted, reducing how much virus recipients inhaled.

Taken together, the findings point to coughing and sneezing as key drivers of influenza spread, particularly from people who shed large amounts of virus, sometimes described as super spreaders. Immunity in those exposed and air movement in indoor spaces also appear to be crucial.

The study does not suggest that influenza is harmless or difficult to catch. Each year, millions, and possibly billions, of cases occur worldwide, with strong evidence that aerosol transmission plays a central role. Instead, it shows that the circumstances that allow flu to spread are more nuanced than simply sharing a room with an infected person.

Not everyone sheds virus at the same level and not everyone is equally vulnerable. Aerosol spread is most likely during coughing and sneezing, so people with these symptoms should isolate where possible and wear a well-fitted mask to reduce virus release into the air. Good ventilation and air circulation are especially important in small, poorly ventilated spaces.

When in doubt, it is safest to assume you could either catch or spread flu and to follow public health guidance, including vaccination and mask use where appropriate.

Conor Meehan does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.