The flu is a bit of a stealth microbe. Infected people may shed significant amounts of flu virus simply by breathing, scientists have discovered.

Every flu season feels bad, but this year could be shaping up to be something more serious.  Public health authorities are concerned because this year's vaccine seems to be less effective than normal, and that could mean more people catching the flu and, unfortunately, more fatalities among vulnerable people.

You've probably heard the conventional wisdom for avoiding the flu: Stay away from infected people who are coughing and sneezing, and wash your hands often to avoid picking up the virus from contaminated surfaces. But a new study suggests these preventative measures don't go far enough. 

The experiment

Dr. Donald Milton is the lead scientist on the study. He is a professor of environmental health at the University of Maryland's School of Public Health in College Park. He had people who were sick with the flu come into his lab and sit in a special booth with a cone in front of their faces like an exhaust hood. "It sucked the air from around them at a reasonably high rate, so that we could capture all the aerosol droplets that they are generating."

Research paper in the journal PNAS

Milton and his team conducted 218 half-hour sessions with 142 confirmed cases of influenza-infected individuals. During each session, the researchers asked the test subjects to recite the alphabet three times. The goal was to determine how many infectious viral particles the infected people emitted when they coughed, sneezed, and breathed.

'We were seeing lots of virus - maybe a thousand particles in half an hour.'- Dr. Donald Milton

"We had designed this instrument to enable us to grow the droplets by vapour condensation on them that get them big enough to make them easy to collect. And we then collected them in a reservoir with plenty of buffer and protein to try to preserve the infectiousness of the virus, and then took them right from the clinic up to the lab and plated them on permissive cells, so that we could see whether they could infect the cells."

Scientists collect all breath from flu-infected people for 30 minutes

As expected, Milton says when people cough, they shed many infectious viruses. "We saw pretty much linear response - the more people were coughing, the more they were shedding. And we saw people shedding up to about 10 million virus copies in a half an hour."

But that's not all they found.

A person infected with the flu who's not even coughing still emits fine aerosolized particles in their breath by simply sitting there, breathing and talking. "We were seeing lots of virus — maybe a thousand particles in some cases in a half an hour."

Milton and his colleagues didn't register many sneezes throughout their sessions. And the sneezes they did pick up didn't result in any additional viruses being shed.

A few of the test subjects came back again, so Milton could test how the progression of their symptoms and infection might affect the number of viruses they shed. He says, "We saw it decline from the first day after symptoms started to the third day, which was as far out as we followed them. They still had virus there but it was declining steadily."

The impact of one virus

"There is some data from experiments done in the 1960s," says Milton, "where back in the bad old bio-warfare days, the Army was trying to see how much virus it took to infect people. And they found that when they had people inhale even a single virus particle, deep into the lung, that was enough to cause infection and full-blown influenza. But when they put the virus in the nose with nose drops, it took several hundred virus particles to get them to make an antibody response and they usually didn't have any symptoms at all."

What you can do to protect yourself

You might think one way to not breathe in other people's flu viruses is to protect yourself by wearing a mask, but that's not what Milton found. "We found in a previous study that when we had people wear a surgical mask for half an hour and then take it off and not wear it for half an hour, and we collected two separate samples, that surgical masks were very good at getting rid of the large droplets. But the fine particles, it only cut them about in half."

The only way to truly protect yourself from influenza-infected individuals is to stay away from them, especially in poorly ventilated areas. Milton says liberal sick leave policies in the workplace will go a long way to keep people in the workplace healthy.

"I think that from an employer point of view, you want to have a policy that when people have flu-like symptoms — they have a fever, muscle aches, fatigue, and a sore throat plus or minus some amount of cough — please tell them to stay home."

To avoid the coarse particles flu-infected people spew when they cough, the best advice is still to wash your hands. Or if you're the sick one, cough into your elbow. But when it comes to the fine, aerosolized particles sick people emit when they breathe, a little avoidance will go a long way.