Flu ‘super-spreaders’: High viral load increases transmission

January 12th, 2015

Just what is the risk of transmission from a coughing patient with influenza? Researchers still can’t answer that question definitively, but airborne particles appear to play a greater role than previously believed.

Werner Bischoff, MD, PhD, FSHEA, an epidemiologist at Wake Forest Baptist Medical Center in Winston Salem, NC, conducted air sampling in the emergency department and inpatient units around 61 patients with confirmed influenza. He detected aerosolized influenza around 26 of the patients – but five of them were “super-spreaders.”

“The higher your nasopharyngeal viral load, the more likely you were to spread influenza into the environment,” he said. “There were five that were spreading exceptionally high amounts of influenza into the environment.”

Bischoff also notes that the distinction between droplets and aerosol particles may not be relevant. “Everyone is emitting a wide range of particle sizes,” he says.

Lindsley used air sampling in a hospital emergency department and urgent care clinic. “We detected flu in every location we monitored,” he said. “We found it throughout the clinic.”

Many of the particles were less than four microns in size, he said. “These are particles that can stay airborne for a long time, they’re easy to inhale and they’re respirable – they can get down deeper in the lung,” he said, though he noted that it wasn’t possible to determine if the small particles were infectious or what the infectious dose would be.

Lindsley also sampled the air around 47 coughing patients with influenza. He found that 84% spread flu aerosols – but 39% of all the influenza RNA detected came from just three patients. “Some patients may be much, much better at spreading influenza around than others are,” he said.

Tests with a coughing machine also revealed what happens to the plume. “No matter where you are in that room, within five minutes, you’re exposed to that coughing plume,” he said.

These findings may point to another important protection for workers, researchers said – putting a mask on coughing patients as source control.

--Michelle Marill