The timing of COVID-19 transmission

  1. Luca Ferretti
  2. Alice Ledda
  3. Chris Wymant
  4. Lele Zhao
  5. Virginia Ledda
  6. Lucie Abeler-Dörner
  7. Michelle Kendall
  8. Anel Nurtay
  9. Hao-Yuan Cheng
  10. Ta-Chou Ng
  11. Hsien-Ho Lin
  12. Rob Hinch
  13. Joanna Masel
  14. A. Marm Kilpatrick
  15. Christophe Fraser

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Abstract

The timing of SARS-CoV-2 transmission is a critical factor to understand the epidemic trajectory and the impact of isolation, contact tracing and other non-pharmaceutical interventions on the spread of COVID-19 epidemics. We examined the distribution of transmission events with respect to exposure and onset of symptoms. We show that for symptomatic individuals, the timing of transmission of SARS-CoV-2 is more strongly linked to the onset of clinical symptoms of COVID-19 than to the time since infection. We found that it was approximately centered and symmetric around the onset of symptoms, with three quarters of events occurring in the window from 2-3 days before to 2-3 days after. However, we caution against overinterpretation of the right tail of the distribution, due to its dependence on behavioural factors and interventions. We also found that the pre-symptomatic infectious period extended further back in time for individuals with longer incubation periods. This strongly suggests that information about when a case was infected should be collected where possible, in order to assess how far into the past their contacts should be traced. Overall, the fraction of transmission from strictly pre-symptomatic infections was high (41%; 95%CI 31-50%), which limits the efficacy of symptom-based interventions, and the large fraction of transmissions (35%; 95%CI 26-45%) that occur on the same day or the day after onset of symptoms underlines the critical importance of individuals distancing themselves from others as soon as they notice any symptoms, even if they are mild. Rapid or at-home testing and contextual risk information would greatly facilitate efficient early isolation.

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    SciScore for 10.1101/2020.09.04.20188516: (What is this?)

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    NIH rigor criteria are not applicable to paper type.

    Table 2: Resources

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  2. Version published to 10.1101/2020.09.04.20188516v2 on medRxiv
  3. Version published to 10.1101/2020.09.04.20188516v1 on medRxiv