Rapid spread of Covid-19 virus has been attributed in large part to its ease of transmission from person to person even before symptoms develop, particularly since an estimated 18% to 75% of patients testing positive for Covid-19 have no symptoms. ^1-4

Transmission before onset of symptoms (presymptomatic): Modeled estimates for the percentage of transmissions that occur from presymptomatic patients range from 37% to as high as 62% based on studies of patients in the cities of Tianjin and Guangzhou in China, as well as Singapore.^5-7 Infectiousness appears to begin within 1-3 days prior to symptoms.^8-10

Transmission when symptoms never develop (asymptomatic): Asymptomatic transmission was invoked in a familial cluster in Anyang, China where 5 patients developed Covid-19 after a 6th asymptomatic family member returned home from Wuhan, China. The asymptomatic patient never developed symptoms—such as fever or respiratory symptom— and had a normal chest CT, but briefly tested positive for Covid-19 by RT-PCR before testing negative later.¹¹

It’s important to point out that up to ~75% of patients who are initially “asymptomatic” later develop symptoms. ^12-14 So what we often call “asymptomatic” may actually be “presymptomatic.”

Transmission of Covid-19 before onset of symptoms is in distinct contrast to SARS, another coronavirus disease, which was transmitted only when a person was symptomatic and was easier to control. This unique property among coronaviruses may be explained by the high tropism of Covid-19 virus not only for the lungs (as in case of SARS virus) but also for the upper respiratory tract.^15,16 As such, Covid-19 behaves more like influenza viruses whose upper respiratory tract binding is thought to promote their rapid transmission even before symptoms develop.¹⁷  No wonder, Covid-19 spread like wild fire!

Coauthor, Bruce Tiu, Harvard Medical Student, Boston, MA

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References

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3. Hu Z, Song C, Xu C, et al. Clinical characteristics of 24 asymptomatic infections with COVID-19 screened among close contacts in Nanjing, China. Sci China Life Sci 2020 Mar 4. https://www.ncbi.nlm.nih.gov/pubmed/32146694
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7. Ganyani T, Kremer C, Chen D, et al. Estimating the generation interval for COVID-19 based on symptom onset data. medRxiv. https://www.medrxiv.org/content/10.1101/2020.03.05.20031815v1
8. Wei WE, Li ZB, Chiew CJ, et al. Presymptomatic transmission of SARS-CoV-2 — Singapore, January 23–March 16, 2020. MMWR Morb Mortal Wkly Rep. ePub: 1 April 2020. https://www.cdc.gov/mmwr/volumes/69/wr/mm6914e1.htm
9. He X, Lau E, Wu P, et al. Temporal dynamics in viral shedding and transmissibility of COVID-19. medRxiv. https://www.medrxiv.org/content/10.1101/2020.03.15.20036707v2
10. Rothe C, Schunk M, Sothmann P, et al. Transmission of 2019-nCoV Infection from an Asymptomatic Contact in Germany. N Engl J Med. 2020;382(10):970–971. https://www.nejm.org/doi/full/10.1056/NEJMc2001468
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12. Kimball A, Hatfield KM, Arons M, et al. Asymptomatic and Presymptomatic SARS-CoV-2 Infections in Residents of a Long-Term Care Skilled Nursing Facility — King County, Washington, March 2020. MMWR Morb Mortal Wkly Rep. 2020;69:377–381 https://www.cdc.gov/mmwr/volumes/69/wr/mm6913e1.htm
13. Chen, C. “What We Need to Understand About Asymptomatic Carriers if We’re Going to Beat Coronavirus”. ProPublica. 2020. https://www.propublica.org/article/what-we-need-to-understand-about-asymptomatic-carriers-if-were-going-to-beat-coronavirus
14. WHO. Report of the WHO-China Joint Mission on Coronavirus Disease 2019 (COVID-19). 2020. https://www.who.int/docs/default-source/coronaviruse/who-china-joint-mission-on-covid-19-final-report.pdf
15. Woelfel R, Corman VM, Guggemos W, et al. Clinical presentation and virological assessment of hospitalized cases of coronavirus disease 2019 in a travel-associated transmission cluster. medRxiv. https://www.medrxiv.org/content/10.1101/2020.03.05.20030502v1
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17. van Riel D, den Bakker MA, Leijten LM, et al. Seasonal and pandemic human influenza viruses attach better to human upper respiratory tract epithelium than avian influenza viruses. Am J Pathol. 2010;176(4):1614–1618. https://wwwnc.cdc.gov/eid/article/26/6/20-0357_article

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