The flu virus might be spread not only by coughing and sneezing, but also simply by breathing, researchers say in a new report.
They found clear evidence that influenza patients breathe the virus out through their mouths and noses in tiny particles that can stay suspended in the air for minutes or hours.
It's useful information in a flu season that has peaked across the continental United States all at once. The Centers for Disease Control and Prevention reported Friday that influenza activity continues on the upswing, with the whole country experiencing the annual influenza epidemic at the same time. So far this season, flu has killed 30 children, the CDC said.
What the new study suggests is that people may need to do more than just wash their hands and keep a distance from sneezing and coughing people to avoid catching the flu.
"We found that flu cases contaminated the air around them with infectious virus just by breathing, without coughing or sneezing," said Dr. Donald Milton, who's been studying flu transmission at the University of Maryland's school of public health.
"Even if you are not coughing, you can still infect other people," he added.
"Many people shedding virus into the air are shedding real, infectious virus."
Milton's team recruits mostly college students who show up at the school clinic with symptoms and get diagnosed with flu. In his lab, he has a "Gesundheit II" machine that collects particles that people sneeze, cough and breathe out so they can be analyzed.
The team got useful data from 178 flu patients who agreed to sit with their faces in the device for half an hour. They mostly sat and breathed, unless they also happened to sneeze, and to see if speaking out loud generated flu-laden particles, they were asked to recite the alphabet a few times.
"Even if you are not coughing, you can still infect other people."
Sneezes not to blame for spread?
The flu patients generated a cloud of particles loaded with virus — some large, some small and some very small. Large particles tend to fall straight to surfaces and that's one well-known way that flu and other infectious diseases spread.
Smaller particles spray out, and the smallest particles, called fine particles, can stay suspended in the air for a while.
The team detected influenza virus from 76 percent of the fine aerosol particles they tested and 40 percent of the coarser particles. they cultured virus from 39 percent of the fine aerosols.
"Thus, sneezing does not appear to make an important contribution to influenza virus shedding in aerosols. Sneezing might make a contribution to surface contamination," the team wrote in their report, published in the Proceedings of the National Academy of Sciences.
Milton says the findings will be controversial. "Right now, the scientific community is just not on the same page about this," he said.
The CDC advises that flu spreads in droplets when people cough, sneeze or talk, and that they travel for a distance of about 6 feet.
"These droplets can land in the mouths or noses of people who are nearby or possibly be inhaled into the lungs. Less often, a person might also get flu by touching a surface or object that has flu virus on it and then touching their own mouth or nose," the CDC says.
Different viruses spread in different ways. Measles is extremely infectious and can be spread in the air even hours after an infected person has left the room. Ebola, on the other hand, is not easily spread and requires close contact with an infected person's body fluids.
This study, combined with others, suggests flu can also spread like measles does — in much smaller droplets suspended in the air.
"You can generate infectious aerosols with breathing. That is important for people to know."
The next step is to close up two people, one healthy, one infected, in a room to see if people can be infected at a distance. Milton's doing a real-life experiment using dormitory roommates.
Allison Aiello, a professor of epidemiology at the University of North Carolina, also studies how flu spreads and says the findings sound reasonable.
"You can generate infectious aerosols with breathing. That is important for people to know," said Aiello, who was not involved in the study.
"We know that respiratory infections are easily transmitted," she added.
While a lot is known about how viruses such as measles spread, flu is harder to study, she said. That's in part because there are so many strains, and the virus is prone to mutations that change its virulence and transmissibility.
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'It's still worth getting the vaccine'
Aiello says she carries a surgical mask to pop on in airplanes and other close quarters when she hears people coughing and sneezing. She said she might wear it more often now, based on the findings.
All experts on flu say washing hands frequently is a very important way to protect yourself and others. So is staying home when sick.
A third factor is air exchange. Forced-air heating and cooling systems pull away old air, and the viruses it might be carrying, and filter the air clean. Airplane ventilation systems do the same.
Milton and Aiello said fogged-up windows in, say, gyms or other closed rooms indicate inadequate air exchange and could be a cue for the germ-averse to flee.
And even in a year when the flu vaccine is not extremely effective, Milton advises being immunized.
"It's still worth getting the vaccine because you are less likely to end up dead or in the hospital," he said.