"The textbook version of contact-tracing starts with someone testing positive for COVID-19 and isolating themselves. A contact-tracer interviews this person to find out who they might have exposed while infected, usually from 48 hours before the positive test, or before symptoms appeared (if there were any). Close contacts — those who’ve spent more than 15 minutes close to the infected person — are of special interest, but anyone who shared public transport or an office space might qualify. Tracers then call or visit those contacts to tell them they need to quarantine, so that they don’t pass the virus on to more people. The chain of transmission is broken. In reality, failures occur at every stage of this test–trace–isolate sequence. People get COVID-19 and don’t know it, or delay getting tested. Positive results can take days to be confirmed. Not everyone who tests positive isolates when requested; one survey in May found that in the United Kingdom, 61% of people who were self-isolating said they’d left their house in the past day. People can’t always be reached for an interview or don’t provide details of their close contacts. And not all contacts are reached, or are willing to comply with quarantine orders."