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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