According to a new study published in the Proceedings of the National Academy of Sciences, it is alarmingly easy to spread the influenza virus by simply breathing.
Colorized transmission electron micrograph of negatively stained SW31 (swine strain) influenza virus particles. Image credit: NIAID / CC BY 2.0.
People commonly believe that they can catch the flu by exposure to droplets from an infected person’s coughs or sneezes or by touching contaminated surfaces.
But the new study about how flu spreads reveals that an infected person may pass the infectious virus to others just by breathing.
“We found that flu cases contaminated the air around them with infectious virus just by breathing, without coughing or sneezing,” said senior author Professor Donald Milton, from the University of Maryland School of Public Health.
“People with flu generate infectious aerosols (tiny droplets that stay suspended in the air for a long time) even when they are not coughing, and especially during the first days of illness.”
“So when someone is coming down with influenza, they should go home and not remain in the workplace and infect others.”
“Our results show that some people can exhale or cough high quantities of live virus particles — up to 1,000 in 30 minutes,” said co-author Dr. Jovan Pantelic, also from the University of Maryland School of Public Health.
“When we consider that an infectious dose is just one virus particle, this means that potentially 1,000 people can be infected in short order.”
“This is especially important in buildings where people sit near each other and especially on public transportation, where the density of people is very high and the air usually is not fresh.”
In the study, the researchers captured and characterized influenza virus in exhaled breath from 142 confirmed cases of people with influenza during natural breathing, prompted speech, spontaneous coughing, and sneezing, and assessed the infectivity of naturally occurring influenza aerosols.
The participants provided 218 nasopharyngeal swabs and 218 30-minute samples of exhaled breath, spontaneous coughing, and sneezing on the first, second, and third days after the onset of symptoms.
The analysis of the virus recovered from these samples showed that a significant number of flu patients routinely shed infectious virus, not merely detectable RNA, into aerosol particles small enough to present a risk for airborne transmission.
Surprisingly, 11 (48%) of the 23 fine aerosol samples acquired in the absence of coughing had detectable viral RNA and 8 of these 11 contained infectious virus, suggesting that coughing was not necessary for infectious aerosol generation in the fine aerosol droplets.
In addition, the few sneezes observed were not associated with greater viral RNA copy numbers in either coarse or fine aerosols, suggesting that sneezing does not make an important contribution to influenza virus shedding in aerosols.
“CDC recommendations include hand washing to protect against contact transmission, but the airborne mode of infection has not been considered as important,” Dr. Pantelic said.
“Our study changes this perception and shows that the airborne mode is very important. We need to consider it and design a guideline for it.”
“The study findings suggest that keeping surfaces clean, washing our hands all the time, and avoiding people who are coughing does not provide complete protection from getting the flu. Staying home and out of public spaces could make a difference in the spread of the influenza virus,” said Sheryl Ehrman, from the Charles W. Davidson College of Engineering at San José State University and the University of Maryland’s Clark School of Engineering.
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Jing Yan et al. Infectious virus in exhaled breath of symptomatic seasonal influenza cases from a college community. PNAS, published online January 18, 2018; doi: 10.1073/pnas.1716561115
