Skip to main content

Quantifying SARS-CoV-2 transmission suggests epidemic control with digital contact tracing

Our take —

This study estimated the generation time (the time between the source and recipient infections) for 40 source-recipient pairs. Using data on symptom onset, authors estimate the proportion of transmission by a) asymptomatic transmission, b) pre-symptomatic transmission, c) symptomatic, and d) environmental transmission. A large proportion of transmission appears to occurs by pre-symptomatic individuals, suggesting that manual contact tracing may not be an effective intervention strategy alone. Technology-based approaches to contact tracing could help identify contacts faster, but could also create privacy concerns, and it is unclear how these tech solutions would link up with public health efforts to support isolation and quarantine.

Study design


Study population and setting

The authors used publicly available information on transmission pairs (from China, Germany, Italy, Singapore, South Korea, Taiwan, and Vietnam). Data were included if there was a high confidence of a true transmission event, and the timings of symptom onset was known for both the source and recipient infection.

Summary of Main Findings

This study estimated the generation time (the time between the source and recipient infections) for 40 source-recipient pairs that were chosen for their high confidence in direct transmission and the known times of symptom onset for both individuals. This was combined with information on the dates of symptom onset and intervals of exposure to estimate the proportion of transmission (measured by R0, the reproductive number) by a) asymptomatic individuals, b) pre-symptomatic (those who will become symptomatic but prior to showing symptoms), c) symptomatic, and d) environmental transmission. Using these estimates, the authors find that the majority of transmission occurs from the pre-symptomatic and symptomatic periods. The contribution to R0 by pre-symptomatic transmission alone was estimated to be 0.9 which is almost at the critical value (R0=1) that will lead to ongoing transmission. The authors then put these values in context of case isolation and contact quarantining which are key non-pharmaceutical interventions being deployed. These results suggest that manual contact tracing and quarantining will not be effective at containing the outbreak since a large proportion of transmission would occur prior to when case isolation and contact tracing could be achieved. These delays make traditional contact tracing less advantageous, and the authors recommend that additional avenues, such as technology-based contact tracing may be a more effective intervention. The authors provide a framework for this type of digital contact tracing and highlight limitations and opportunities for this type of intervention (such as privacy concerns, adoption for high risk groups, and the need for open and clear communication about the algorithm used).

Study Strengths

This study directly estimated the generation time from transmission pairs with a high confidence of direct transmission where the time of symptom onset for both the source and recipient were known. They highlight the importance of designing contact-tracing and quarantining interventions that take into account transmission that likely occurred prior to symptom onset.


The framework used is reliant on known transmission pairs which may limit the generalizability of these results in areas with higher community transmission. Although the authors suggest that the use of a technology-based app that could quickly tell someone if they have recently been in contact with a case, they do not provide additional information on how that might be implemented or adopted by the general population, or how it would link up with other ongoing contact tracing efforts. The authors do not fully address concerns about adoption, privacy, and representativeness of this approach and the subsequent implications on the effectiveness.

Value added

This study provides estimates of the contribution of overall transmission from asymptomatic, pre-symptomatic, symptomatic, and environmental transmission using detailed data on source-recipient infections. These results suggest that interventions that depend on first identifying symptomatic individuals and their contacts will result in delays that greatly diminish its effectiveness.