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Last weekend, in his Virology Down Under blog, Dr. Ian Mackay took a long look at the ways humans shed and spread influenza viruses (see Influenza virus transmission: with or without symptoms, you’re dropping Flu virus). It is an excellent review, well worth reading in its entirety,
Ian looked at both large droplet and fine aerosol transmission from coughing, sneezing and simply just breathing - along with fomite contamination - even from asymptomatic carriers.While viral RNA had been detected simply from the exhalation of flu victims, Ian cautioned:
Viruses were not able to be grown in culture, or culture was not used in these studies and this is a limitation because we can’t say with certainty that viruses were breathed out during such studies could infect a susceptible person; we don’t know if the positive results mean infectious virus was present.This is a topic I wrote about a couple of weeks ago myself, while reviewing a Journal of Infectious Disease study (see J.I.D.: Asymptomatic Summertime Shedding Of Respiratory Viruses).
Well science marches on, and a new study published yesterday in the Journal PNAS (Proceedings of the National Academy of Science) provides additional evidence for, and gives considerable more weight to, the idea that flu carriers exhale substantial quantities of infectious influenza virus.
Infectious virus in exhaled breath of symptomatic seasonal influenza cases from a college community
Jing Yana,b, Michael Granthama,1, Jovan Pantelica,2, P. Jacob Bueno de Mesquitaa, Barbara Alberta, Fengjie Liua,3, Sheryl Ehrmanb,4, Donald K. Miltona,5, EMIT Consortium6
Lack of human data on influenza virus aerosol shedding fuels debate over the importance of airborne transmission. We provide overwhelming evidence that humans generate infectious aerosols and quantitative data to improve mathematical models of transmission and public health interventions.
We show that sneezing is rare and not important for—and that coughing is not required for—influenza virus aerosolization. Our findings, that upper and lower airway infection are independent and that fine-particle exhaled aerosols reflect infection in the lung, opened a pathway for a deeper understanding of the human biology of influenza infection and transmission. Our observation of an association between repeated vaccination and increased viral aerosol generation demonstrated the power of our method, but needs confirmation.
Little is known about the amount and infectiousness of influenza virus shed into exhaled breath. This contributes to uncertainty about the importance of airborne influenza transmission.
We screened 355 symptomatic volunteers with acute respiratory illness and report 142 cases with confirmed influenza infection who provided 218 paired nasopharyngeal (NP) and 30-minute breath samples (coarse >5-µm and fine ≤5-µm fractions) on days 1–3 after symptom onset. We assessed viral RNA copy number for all samples and cultured NP swabs and fine aerosols.
We recovered infectious virus from 52 (39%) of the fine aerosols and 150 (89%) of the NP swabs with valid cultures. The geometric mean RNA copy numbers were 3.8 × 104/30-minutes fine-, 1.2 × 104/30-minutes coarse-aerosol sample, and 8.2 × 108 per NP swab. Fine- and coarse-aerosol viral RNA were positively associated with body mass index and number of coughs and negatively associated with increasing days since symptom onset in adjusted models.
Fine-aerosol viral RNA was also positively associated with having influenza vaccination for both the current and prior season. NP swab viral RNA was positively associated with upper respiratory symptoms and negatively associated with age but was not significantly associated with fine- or coarse-aerosol viral RNA or their predictors. Sneezing was rare, and sneezing and coughing were not necessary for infectious aerosol generation. Our observations suggest that influenza infection in the upper and lower airways are compartmentalized and independent.(Continue . . . )
The full, open access study can be read here, and it contains a number of gems.
- Men shed influenza viruses in greater quantity than women through fine aerosols.
- But women cough more frequently
- Most surprisingly, they observed `6.3 (95%
CI 1.9–21.5) times more aerosol shedding among cases with vaccination
the current and previous season compared
with having no vaccination in those two seasons.'
In addition to releasing a short video, the University of Maryland School of Public Health published the following press release (see excerpt below) on this new study. You'll also find a pretty good picture of the Gesundheit II machine they used in their study on their site.
Flu may be spread just by breathing, new study shows; coughing and sneezing not required
January 18, 2018
It is easier to spread the influenza virus (flu) than previously thought, according to a new University of Maryland-led study released today. 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, new information about flu transmission reveals that we may pass the flu 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,” explained Dr. Donald Milton, M.D., MPH, professor of environmental health in the University of Maryland School of Public Health and lead researcher of this study. “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.”
Researchers from the University of Maryland, San Jose State University, Missouri Western State University and University of California, Berkeley contributed to this study funded by the Centers for Disease Control and Prevention (CDC) and the National Institutes of Health.
(Continue . . . )
All of these tests were conducted on symptomatic, flu positive cases and illustrate why it is important to stay home when you are sick, even if you aren't coughing or sneezing up a storm.
What may be a `mild flu' for you could easily be deadly for someone else.As we've discussed previously, people can spread influenza during the 24 hours before symptoms appear - or may have such minor symptoms as to not realize they are ill - (see PLoS One: Influenza Viral Shedding & Asymptomatic Infections).
While asymptomatic spread still provides a loophole for the virus to spread, this study lends additional support to the idea of having and wearing simple surgical masks when you are home with the flu and are around other family members - even if you aren't coughing or sneezing.
But most of all, this study shows us just how easily influenza can spread in the community, even from someone who doesn't appear all that sick.