Does contact tracing work? Quasi-experimental evidence from an Excel error in England (PDF) CAGE Research Centre. A natural experiment. From the Abstract: “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 to the contact tracing system. We show that 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 test 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. Our findings provide strong quasi-experimental evidence for the effectiveness of contact tracing.”
It thought itself very successful in the beginning mainly because it underestimated and underpriced risk. After a decade or so it discovered that it hadn’t really understood what it was doing and began taking on large losses, after which it cut back sharply.