COVID-19: 6% of all SARS-CoV-2 Transmissions May be Presymptomatic

Miriam Davis, PhD

April 09, 2020


  • Presymptomatic transmission of SARS-CoV-2 may account for >6% of 157 locally acquired cases of COVID-19 in Singapore.

  • The transmission occurred 1-3 days before symptom onset.

Why this matters

  • Viral shedding may happen in the absence of symptoms and before symptom onset.

  • To prevent transmission, it may not be enough for only people with symptoms to limit their contacts.

  • Findings support social distancing and wearing of surgical masks by the population to control the pandemic.

Study design

  • Contact tracing of all 243 cases of COVID-19 in Singapore, 157 of them locally acquired.

  • Presymptomatic transmission was defined by SARS-CoV-2 transmission from an infected person (the source patient) to another patient before the source patient developed symptoms, with no evidence that the secondary patient had been exposed to anyone else with COVID-19.

  • Funding: None disclosed.

Key results

  • A total of 7 epidemiological clusters were identified in which presymptomatic transmission occurred.

  • 10 of 157 locally acquired cases (6.4%) within these clusters involved presymptomatic transmission.

  • Presymptomatic transmission occurred 1-3 days before symptom onset in 4 clusters with an identifiable date of exposure.


  • An unknown source might have initiated the clusters.

  • Potential for recall bias.

  • Observational.

Wei WE, Li Z, Chiew DJ, Yong SE, Toh MP, Lee VJ. Presymptomatic Transmission of SARS-CoV-2 — Singapore, January 23–March 16, 2020. MMWR Morb Mortal Wkly Rep. 2020 Apr 1 [Epub ahead of print]. doi: 10.15585/mmwr.mm6914e1.
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This clinical summary first appeared on Univadis from Medscape.


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