SARS-CoV-2: The second wave of COVID-19


The outbreak of coronavirus disease 2019 (COVID-19), which began in Wuhan, China, in late 2019, has spread to 203 countries as of March 30, 2020, and has been officially declared a global pandemic.

With unprecedented public health interventions, local transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) appears now to have been contained in China. Multiple countries are now experiencing the first wave of the COVID-19 epidemic; thus, gaining an understanding of how these interventions prevented the transmission of SARS-CoV-2 in China is urgent.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the viral strain that causes coronavirus disease 2019 (COVID-19), a respiratory illness. It is colloquially known as the coronavirus, and was previously referred to by its provisional name 2019 novel coronavirus (2019-nCoV).


  • 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.
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