Free DP Download 03 December 2020 - DOES CONTACT TRACING WORK? Quasi-Experimental Evidence from an Excel Error in England

Thursday, December 3, 2020

Thiemo Fetzer, Thomas Graeber  
CEPR DP No. 15494 November 2020

Despite numerous challenges faced by a contact tracing system, such as a people’s lack of trust, non-adherence and privacy concerns, this non-pharmaceutical intervention can have a strong impact on the progression of an infectious disease such as Covid-19. 

This is the main finding of a new CEPR study by Thiemo Fetzer and Thomas Graeber, which evaluates the effectiveness of contact tracing by exploiting a quasi-random variation in Covid-19 contact tracing in England. 

Between September 25 and October 2, 2020, a total of 15,841 COVID-19 cases in England (around 15 to 20% of all cases) were not immediately referred to the contact tracing system due to a data processing error. Case information was truncated from an Excel spreadsheet after the row limit had been reached, which was discovered on October 3. There is substantial variation in the degree to which different parts of England areas were exposed -- by chance -- to delayed referrals of Covid-19 cases to the contact tracing system. The findings revealed:

  • More affected areas subsequently experienced a drastic rise in new Covid-19 infections and deaths alongside an increase in the positivity rate and the number of tests performed, as well as a decline in the performance of the contact tracing system. 
  • Conservative estimates suggest that the failure of timely contact tracing due to the data glitch is associated with more than 125,000 additional infections and over 1,500 additional Covid-19-related deaths. 
  • These novel findings provide strong quasi-experimental evidence for the effectiveness of contact tracing, at a time when evidence about their effectiveness remains scarce.

Panel D: Percentage of cases not contact traced

Notes: The geographic distribution of the fraction of cases tested from Sept 20 to Sept 27 that were not referred to contact tracing until Oct 3 or Oct 4.

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