Flu spreads via airborne droplets

Half of flu cases arise when people inhale tiny particles that float in the air, an international group of researchers reports June 4 in Nature Communications. The finding flies in the face of conventional wisdom, which holds that nearly all influenza spreads by large droplets that sick people release when they sneeze or cough. Those large droplets, the theory went, get on people’s hands and transmit the virus from there.

While scientists knew that small particles called aerosols represent possible routes of disease spread, they thought that cases almost never arise that way.

Public health officials say that knowing how often flu transmits via the air is important for controlling outbreaks, especially when dealing with pandemic strains for which no vaccine exists.

Benjamin Cowling, an infectious disease epidemiologist at the University of Hong Kong, and his colleagues studied how flu spread among 782 families in Bangkok and Hong Kong during regular outbreaks in 2008 through 2011. Some  families received liquid soap and instructions about proper hand hygiene. Some also got surgical masks. Together, washing hands and wearing surgical masks should block transmission of the virus through the large droplet route.

But surgical masks don’t block airborne flu. So the researchers assumed that people who got sick even though they wore the masks and washed their hands probably caught the flu by inhaling small particles. And to work out how the families without soap and masks caught the flu, the researchers further assumed, based on previous studies, that people who got sick through the airborne route were more likely to develop classic flu symptoms such as fevers and coughs. People who catch flu via large droplets typically get milder symptoms.

Aerosol transmission caused 33 percent to 92 percent of cases in Hong Kong and 55 percent to 98 percent in Bangkok, the team calculated.

“There’s a lot of uncertainty in that data,” says Donald Milton, an environmental health scientist at the University of Maryland’s School of Public Health in College Park. But the exact proportion of cases due to airborne spread doesn’t matter as much as the evidence that aerosols are an important route of transmission, he says. “What it really says is you can’t rule out aerosols.”