Tiny particles that float in the air may contribute to the spreading of flu, a modelling study suggests.

Seasonal influenza A viruses are thought to spread when people are exposed to large respiratory droplets in coughs and sneezes from those who are sick. Those large droplets fall to the ground within a couple of metres.

In contrast, a larger number of smaller aerosolized droplets that are too tiny to be seen with the naked eye may also transmit flu.

In this week's issue of the journal Nature Communications, Benjamin Cowling of the University of Hong Kong and his colleagues described a mathematical model that predicted aerosol transmission is an important way of spreading flu in households.

"We find that aerosol transmission accounts for approximately half of all transmission events," the researchers concluded.

"This implies that measures to reduce transmission by contact or large droplets may not be sufficient to control influenza A virus transmission in households."

In hospitals, precautions for airborne illnesses include wearing respirators on top of the usual masks, gloves and gown measures for droplets.

The researchers used data from two randomized trials of hand hygiene and face masks in families in Hong Kong and Bangkok to plug into the model. When they did, the hand and face protection did not make a major difference to overall infection risk.

Model questioned by experience

But the findings don't hold up in real life terms, said Dr. Michael Gardam, medical director of infection prevention and control at Toronto’s University Health Network.

"Fundamentally, 50 per cent airborne transmission flies in the face of everything that we know about flu," said Dr. Michael Gardam, medical director of infection prevention and control at Toronto’s University Health Network. "It just doesn't make sense."

Many modelling studies are done on flu, but the definitive experiment of exposing human volunteers to relatively mild flu under carefully controlled laboratory conditions to test which mode of transmission matters most hasn't been done, he said.

"The frustrating thing with all of this is it's not just academic. This actually matters because the control measures that we use are going to be based on how it's transmitted."

H7N9 bird flu evolution

A second paper in this week's New England Journal of Medicine provided a reality check on what's known about H7N9 bird flu in China.

Dr. Anthony Fauci, director of the U.S. National Institute of Allergy and Infectious Diseases, and his co-authors wrote a commentary titled "Pandemic Influenza Viruses — Hoping for the Road Not Taken."

Fauci noted that the H7N9 bird flu virus has some "human-like mutations" that have raised concerns for its pandemic potential. 

But there is no firm scientific evidence that pandemic viruses follow a linear path, Fauci said.

"H7N9's journey has just begun. We can only hope that the road to a pandemic is the road not taken," the paper concluded.

The World Health Organization's web page says little is known about the scope of the disease that the virus causes and sources of exposure.

A total of 132 lab confirmed cases of human infection with H7N9 have been reported since March, the agency said Friday. Of those, 37 people have died and most of the other cases were considered severe.

About three out of four patients report a history of exposure to birds, mostly chickens, WHO said.

The number of new reported cases decreased after Chinese public health officials closed live poultry markets and culled chickens where there were human cases. But there's no way to tell if those measures truly had an impact, Gardam said.

Traditionally, China's flu season ends when temperatures climb during summer in the Northern Hemisphere.

With files from CBC's Amina Zafar