Contact tracing for COVID-19
Nicola Low, Institute of Social and Preventive Medicine, University of Bern, CH
Wednesday 7th October, 2020. WHO/GOARN Global Consultation on Contact Tracing for COVID-19
Forwards and backwards: an introduction
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Contact Tracing for COVID-19: Forwards and Backwards, Summaries of Preventive medicine
The importance of contact tracing in controlling the spread of COVID-19. It explains the concepts of forwards and backwards contact tracing, the role of basic reproduction number, overdispersion, and the impact of contact tracing on identifying secondary infections and reducing the size of superspreading events. Real-life case studies are provided to illustrate the process.
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Contact tracing for COVID- 19
Nicola Low, Institute of Social and Preventive Medicine, University of Bern, CH Wednesday 7th October, 2020. WHO/GOARN Global Consultation on Contact Tracing for COVID- 19
Forwards and backwards: an introduction
What does contact tracing do?
Forward and backward contact
tracing
Case study
Expected impact
Discussion
Outline
Why do we say ‘forwards’ and ‘backwards’?
Contact tracing
Source
Contact
Index
case
I n c u b a t i o n
I n f e c t i o u s
S ym p t o m s
Why do we say ‘forwards’?
Contact tracing
Source
Contact
Index
case
I n c u b a t i o n
I n f e c t i o u s
Pre-symptomatic transmission
Quarantine
Stop forward
transmission
SARS-CoV- 2 transmission
The role of overdispersion
Riou J and Althaus CL. Eurosurveillance. Jan. 2020 Based on Laxminarayan R, et al. Science. Sept. 2020
Overdispersion and contact tracing
Index case is disproportionately likely to have been infected by a source who also infected others in a cluster Drosten C. Die Zeit 33/2020, 06.08.
Index case
Index case might infect few contacts
Forward contact tracing
Day 2 A contact tracer asks the index case about his contacts in the two days before his symptoms started The index case was in close contact with his wife and children. He travelled to an event on a helicopter with his wife and an aide All go into quarantine and are tested
Case study: COVID- 19 outbreak
Photo credits: www.nytimes.com and www.theguardian.com 02.10.
Backward contact tracing
Day 2 The contact tracer asks the index case about his activities in the two weeks before his symptoms started Did he go to any events where he was in contact with people for more than 15 minutes, without masks and without a distance of 1.5m or more? They make an activity map
Case study: COVID- 19 outbreak
Index case Photo credit: www.nytimes.com 04.10.
Backward contact tracing
Day 4- 6 Contact tracers try to identify and contact all the staff at the index case’s home and guests across different states They send all guests into quarantine and ask them to be tested They find 20 additional cases The contact tracers carry on to test, trace, isolate and quarantine A potential superspreading event is avoided
Case study: COVID- 19 outbreak
Photo credit: https://www.washingtonpost.com, 03.10.
What is the impact?
Mathematical model A. Forward contact tracing only Identifies, at most, the mean number of secondary infections B. Forward and backward tracing Increases cases found by factor 2 - 3 Identifies high-risk settings
Forward and backward contact tracing
Endo A, et al. medRxiv preprint https://doi.org/10.1101/2020.08.01. c, reduction in infectiousness due to quarantine