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COVID-19 Clinical Outcomes

COVID-19: The Impact of Asymptomatic Cases on the Pandemic Outbreak

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BACKGROUND AND PURPOSE:

  • Undocumented COVID-19 infections are likely to increase the spread and severity of the pandemic worldwide
    • Undocumented infections can be due to limited testing availability, and cases that are mild or asymptomatic
  • Li et al. (Science, 2020) sought to estimate the contagiousness and proportion of undocumented SARS-CoV-2 infections in China during the weeks before and after the shutdown of travel in and out of Wuhan

METHODS:

  • Mathematical modeling
    • Simulated infection spread throughout time and space across 375 Chinese cities before and after travel restrictions
    • Cases were classified as
      • Documented (confirmed) and undocumented
    • ‘Re’ (effective reproductive number): Number of people each infected person infects
      • If <1, cases will decrease

RESULTS:

Before Travel Restrictions Modeling

  • Virus Characteristics
    • Re: 2.38 (95% CI 2.04 to 2.77)
    • SARS-CoV-2 has high capacity for sustained transmission
    • Median latent period: Approximately 3.69 days
    • Median infectious period: Approximately 3.48 days
  • Undocumented cases
    • Only 14% (95% CI, 10 to 18%) of total infections in China were reported
  • Impact of undocumented cases
    • An estimated 79% of individuals were infected by undocumented cases
    • The authors estimated that without infections from undocumented cases, the infection would have been substantially reduced
      • In Wuhan: 66.1%
      • Across all of China: 78.8%

After Travel Restrictions Modeling

  • Virus characteristics
    • The Re decreased after travel restrictions were enacted
      • January 10 to 23 (before restrictions): 2.38
      • January 24 to February 8: 0.99 (95% CI: 0.76 to 1.33)
    • Estimates of both the latency and infectious periods were similar to those before travel restrictions
  • Undocumented cases
    • The contagiousness of undocumented infections was reduced, likely due to increased testing and effective social distancing

CONCLUSION:

  • Through mathematical modeling, the authors discovered that
    • Approximately 86% of COVID-19 cases prior to China restrictions were undocumented
    • These undocumented cases facilitated rapid spread throughout China
  • In addition, cases have a period of pre-symptomatic shedding
  • The authors assert that to control SARS-CoV-2 there needs to be
    • Major increase in testing
    • Effective isolation of asymptomatic or mild cases
  • Unclear if
    • Re can be maintained at <1 for a sufficient length of time to eliminate the disease locally once containment measures are relaxed
    • International containment measures are required to allow opening of borders, while still preventing reintroduction of the virus

Learn More – Primary Sources:

Substantial undocumented infection facilitates the rapid dissemination of novel coronavirus (SARS-CoV2)

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Related ObG Topics:

COVID-19 in Italy: Why is the Case Fatality Rate So High?
Aerosols and Surfaces: SARS-CoV-2 Stability
Coronavirus and COVID-19: Clinical Characteristics from 138 Patients in Wuhan, China

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