A new study led by Emory University and published in Cell on March 19 reveals that people infected with influenza expel varying amounts of infectious virus into the air, with some individuals releasing orders of magnitude more than others.
The research provides important insight into why some people may be more likely to transmit respiratory viruses like influenza. By capturing live virus directly from the air exhaled by participants in a controlled human infection study, scientists traced viral genetic diversity from inside the body into the environment.
Seema S. Lakdawala, PhD, senior author and associate professor at Emory University, said: “We’ve known influenza can be transmitted through the air, but directly capturing infectious virus from people and linking it to symptoms and viral load over time has been technically very difficult. This study shows that some individuals release orders of magnitude of more infectious virus than others, suggesting a superspreader-like effect. It also shows that the amount of virus in the nose is not the best metric for predicting how much infectious virus is expelled into the air.”
To overcome previous technical barriers, researchers developed a Modular Influenza Sampling Tunnel (MIST) that collects respiratory particles while preserving their infectivity by allowing them to settle onto living cells. Linsey C. Marr, PhD, co-senior author from Virginia Tech, said: “Measuring infectious virus in the air is incredibly challenging. Existing methods are good at counting particles or detecting viral genetic material, but they don’t preserve virus infectivity. We designed our sampling method to bridge that gap so we can connect what people are exhaling with the ability to transmit an infection.”
Using a controlled human infection model allowed researchers to closely monitor participants’ symptoms and airborne shedding over several days. Nadine G. Rouphael, MD from Emory University said: “Human challenge studies provide a unique window into how infections unfold in real people, day by day… This approach allows us to safely and carefully link symptoms, viral dynamics and transmission-relevant outcomes in ways that directly inform public health.”
Key findings include significant differences among individuals’ airborne release of infectious flu; some expelled little or none while others released levels differing by more than three orders of magnitude. The amount expelled correlated most strongly with symptom severity such as congestion and fatigue rather than just coughing or sneezing alone.
Sequencing showed multiple viral genetic variants were present in aerosols released by participants. Anice C. Lowen, PhD at Emory University said: “We were surprised by how much viral diversity was maintained in the particles people expelled… This tells us that tight genetic bottlenecks seen during transmission don’t happen when virus leaves the body—they occur later during exposure of a new host.”
Researchers say these findings help explain why flu transmission can be highly uneven between individuals—a small number may drive much of its spread—and offer data for evaluating interventions like ventilation or masking.
The MIST platform could also help study other respiratory viruses beyond influenza.
