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What’s the role of small droplets or aerosolized particles in the transmission of Covid-19?

FA Manian MD, MPH, , , , , , , , , , , 2 Comments

Although transmission of SARS-CoV-2 is often considered to occur through large respiratory droplets by coughing or sneezing, emerging data suggests that smaller respiratory particles (5 microns or less) generated by breathing, speaking or singing also account for a sizeable number of infections. Several lines of evidence make a cogent argument for aerosols serving as an important mode of transmission for SARS-CoV-2. 1-9

 First, there are ample accounts of SARS-CoV-2 spreading by being near an infected individual without symptoms.  Since by definition, those without symptoms do not cough or sneeze transmission must have occurred through other means, including breathing, talking or touching surfaces that might have become secondarily contaminated through aerosol.1,2,5  To make matters worse, the peak of contagion in infected individuals occurs on or before symptoms occur.1

Second, aerosolized SARS-CoV-2 has been shown to remain viable in the air for at least 3 hours and viral RNA (not necessarily viable virus) has been found in the air outside patient rooms and inside patient rooms in the absence of cough.2,9 One study found SARS-CoV-2 in outdoor air at a hospital entrance and in front of a department store.7

Third, contaminated air samples and long-range aerosol transport and transmission have been reported by several studies involving a related coronavirus, SARS-CoV-1, the agent of SARS.2

What’s the ramifications of aerosol transmission of Covid-19? The most obvious is the requirement for universal wearing of masks or face covers in public spaces even when 6 feet apart. This practice is particularly important indoors where the amount of ventilation, number of people, duration of stay in the facility, and airflow direction may impact the risk of exposure to SARS-CoV-2.1

The other potential ramification of aerosolized SARS-CoV-2 is that due to their smaller size, these virus-laden particles may bypass the upper respiratory tract and be inhaled directly into the lungs resulting in more severe disease.4  So it really makes sense to routinely wear a mask when out in public places.

Bonus Pearl: Did you know that 1 minute of loud speaking could generate over 1000 virus-containing aerosols in the air with a “super-emitter” generating over 100,000 virus particles in their droplets during the same time?1

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 References

  1. Prather KA, Wang CC, Schooley RT. Reducing transmission of SARS-CoV-2. Science. May 27, 2020.
  2. Anderson EL, Turnham P, Griffin JR, et al. Consideration of the aerosol transmission for COVID-19 and public health. Risk Analysis 2020;40:902-7.
  3. Hamner L, Dubbel P, Capron I, et al. High SARS-CoV-2 attack rate following exposure at a choir practice-Skagit County, Washington, March 2020. MMWR 2020; 69: 606-10.
  4. Gralton J, Tovey E, McLaws ML, et al. The role of particle size in aerosolized pathogen transmission: a review. J Infect 2011;62:1-13.
  5. Asadi S, Bouvier N, Wexler AS et al. The coronavirus pandemic and aerosols: does COVID-19 transmit via expiratory particles. Aerosol Sci Technol 2020;54:635-38.
  6. Morawska L, Cao J. Airborne transmission of SARS-CoV-2: the world should face the reality. Env International 2020;139:105730.
  7. Liu Y, Ning Z, Chen Y, e al. Aerodynamic analysis of SARS-CoV-2 in two Wuhan hospitals. Nature 2020;582:557-60. https://www.nature.com/articles/s41586-020-2271-3.pdf
  8. Somsen GA, van Rijn C, Kooij S, et al. Small droplet aerosols in poorly ventilated spaces and SARS-CoV-2 transmission. Lancet Respir Med 2020; May 27. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7255254/pdf/main.pdf

9. Santarpia JL, Rivera DN, Herrera V, et al. Transmission potential of SARS-CoV-2 in viral shedding observed at the University of Nebraska Medical Center. 2020 (Preprint) https://www.ehs.ucsb.edu/files/docs/bs/Transmission_potential_of_SARS-CoV-2.pdf

Disclosures: The listed questions and answers are solely the responsibility of the author and do not necessarily represent the official views of Massachusetts General Hospital, Harvard Catalyst, Harvard University, its affiliate academic healthcare centers, or its contributors. Although every effort has been made to provide accurate information, the author is far from being perfect. The reader is urged to verify the content of the material with other sources as deemed appropriate and exercise clinical judgment in the interpretation and application of the information provided herein. No responsibility for an adverse outcome or guarantees for a favorable clinical result is assumed by the author. Thank you!

What’s the role of small droplets or aerosolized particles in the transmission of Covid-19?

How can people with a respiratory virus such as Covid-19 be contagious even when they don’t cough or sneeze?

FA Manian MD, MPH, , , , , , , , , , , , , , , Leave a comment

Couple of factors likely play a role in the transmission of respiratory viruses such as Covid-19 even in the absence of respiratory symptoms: 1. Generation of small droplets through everyday activities such as talking and breathing; 2. Presence of infectious virus in the respiratory tract before onset of symptoms.1-4

Small droplet generation during every day activity: Normal human speech and breathing can yield small particles or droplets that are too small to see by naked eye but are perfectly capable of serving as vehicles for aerial transport (more like hot air balloons than 737’s!) of a variety of communicable respiratory pathogens. 1  These small particles are believed to originate from the mucosal layers coating the respiratory tract as well as from vocal cord adduction and vibration within the larynx.1

In some cool experiments involving normal volunteers,1 the rate of particle emission during normal human speech positively correlated with the loudness of voice, ranging from 1-50 particles/second, irrespective of the language spoken (English, Spanish, Mandarin, or Arabic).  Perhaps, equally intriguing was identification of “speech superemitters”, consistently releasing an order of magnitude more particles than other participants.

Simply counting out loud has been associated with around 2-10 times as many total particles emitted as a single cough, 2 and the percentage of airborne droplet nuclei generated by singing is several times more than that emitted during normal talking and more like that of coughing! 3 Given, these observations, perhaps, the unfortunate outbreak of Covid-19 among members of a church choir in state of Washington 5 is not totally unexpected.

Presence of infectious virus in persons without symptoms:  An estimated 18% to 75% of patients testing positive for Covid-19 have no symptoms. This of course means that irrespective of whether symptoms ever develop, persons with Covid-19 may serve as a source of infection, by just breathing, talking, or singing when around susceptible people.

For these reasons, social distancing and wearing of masks during a pandemic makes sense!

Bonus Pearl: Did you know that infectious viral particles can be recovered from 40% of breath samples of patients with influenza? 6

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 References

  1. Asadi S, Wexler AS, Cappa CD, et al. Aerosol emission and superemission during human speech increase with voice loudness. Scientific Reports 2019;9:2348. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6382806/
  2. Loudon RG, Roberts RM. Droplet expulsion from the respiratory tract. Am Rev Resp Dis 1967;435-42. https://doi.org/10.1164/arrd.1967.95.3.435
  3. Loudon RG, Roberts MR. Singing and the dissemination of tuberculosis. Am Rev Resp Dis 1968;98:297-300. DOI: 10.1164/arrd.1968.98.2.297 https://www.atsjournals.org/doi/abs/10.1164/arrd.1968.98.2.297?journalCode=arrd
  4. Lai KM, Bottomley C, McNerney. Propagation of respiratory aerosols by the Vuvuzela. PLoS One 2011;6:e20086. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3100331/
  5. Read R. A choir decided to go ahead with rehearsal. Now dozens of members have COVID-19 and two are dead. Los Angeles Times March 29, 2020. https://www.latimes.com/world-nation/story/2020-03-29/coronavirus-choir-outbreak
  6. Yan J, Grantham M, Pantelic J, et al. Infectious virus in exhaled breath of symptomatic seasonal influenza cases from a college community. PNAS 2018;115:1081-1086 https://www.pnas.org/content/115/5/1081

Disclosures: The listed questions and answers are solely the responsibility of the author and do not necessarily represent the official views of Massachusetts General Hospital, Harvard Catalyst, Harvard University, its affiliate academic healthcare centers, or its contributors. Although every effort has been made to provide accurate information, the author is far from being perfect. The reader is urged to verify the content of the material with other sources as deemed appropriate and exercise clinical judgment in the interpretation and application of the information provided herein. No responsibility for an adverse outcome or guarantees for a favorable clinical result is assumed by the author. Thank you!

How can people with a respiratory virus such as Covid-19 be contagious even when they don’t cough or sneeze?