Scientists reveal why some people with the flu may be more contagious

EPFL scientists have found that in closed spaces, droplets containing the flu virus remain infectious for longer if they also contain certain types of bacteria that live in our respiratory tract. This discovery provides important information about how respiratory infections are transmitted and could improve risk assessments.

Even healthy people carry many different types of bacteria on their skin and in their gut, nose, and mouth. When we get a respiratory illness like the flu, the viruses live alongside the bacteria in our respiratory tract. But what happens to these viruses when we sneeze, cough, or talk?

A new study by scientists from the Laboratory of Experimental Virology (LEV) in collaboration with colleagues from EPFL's School of Architecture, Civil and Environmental Engineering (ENAC), ETH Zurich and the University of Zurich examines the behaviour of the influenza virus outside the human body and how this behaviour is influenced by respiratory bacteria.

The team's findings were recently published in the Journal of Virology.

It has long been known that certain types of bacteria in the human gut allow viruses to stabilize and live longer. But Shannon David, a researcher at LEV, wondered whether bacteria from the respiratory tract play a similar protective role in droplets expelled from the human body.

To find out, she and her colleagues conducted two types of lab experiments. In the first, they created droplets similar to those produced by sneezing and placed them on a flat surface exposed to room air. Some droplets contained only the flu virus, while others also contained bacteria commonly found in the respiratory tract.

The scientists allowed the droplets to dry and then measured the infectious viral load over time. They found that after 30 minutes, the bacteria-free droplets had almost completely killed the virus (99.9%). In droplets containing both virus and bacteria, the infectious viral load was 100 times greater at the same time, and the virus could survive for many hours.

In a second type of experiment, the scientists measured the infectious viral load of aerosol droplets. Here, they found that particles containing only the virus were no longer infectious after 15 minutes. But particles containing bacteria also contained the virus after one hour.

The bacteria with the greatest stabilizing effect were Staphylococcus aureus and Streptococcus pneumoniae, both of which commonly colonize the respiratory tract.

Flat drops

Next, the scientists wanted to understand how respiratory bacteria could protect the flu virus outside the human body. They looked at droplet samples under a microscope. “The droplets that contain bacteria tend to be flatter,” says David.

"This speeds up the evaporation process and leads to faster crystallization of the salt in the droplet, allowing the viruses to survive longer. This could be an important factor in dry conditions, such as indoors in winter when the heating is on."

“Until now, little was known about the role that respiratory bacteria play outside the human body,” says David. “These findings provide an important piece of the puzzle about how respiratory diseases are transmitted. And they help explain why viruses spread so easily from person to person.”

The data collected by her team will be useful for research in a number of areas, including public health.

“The models currently used to predict the spread of the virus in closed spaces do not take into account the protective function of bacteria,” says David. “This means they are likely to underestimate the risk of infection.”

This study may allow researchers to more easily identify people who are more likely to shed higher infectious viral loads because they have more protective bacteria in their respiratory tract.