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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- 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/
- Loudon RG, Roberts RM. Droplet expulsion from the respiratory tract. Am Rev Resp Dis 1967;435-42. https://doi.org/10.1164/arrd.1922.214.171.1245
- Loudon RG, Roberts MR. Singing and the dissemination of tuberculosis. Am Rev Resp Dis 1968;98:297-300. DOI: 10.1164/arrd.19126.96.36.1997 https://www.atsjournals.org/doi/abs/10.1164/arrd.19188.8.131.527?journalCode=arrd
- 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/
- 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
- 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
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