When Ebola ripped through communities in West Africa between 2014 and 2016, Tolbert Nyenswah saw at first hand how health workers extinguished the epidemic by finding and quarantining contacts of those who caught the disease. The former director of Liberia’s public-health institute thought contact-tracers would again rise to the challenge this year, keeping COVID-19 in check as it swept the globe. “Contact-tracing is one of the greatest tools that countries should deploy and use effectively to contain the outbreak,” he says.

But nine months after the World Health Organization (WHO) labelled COVID-19 as a pandemic, few countries are wielding contact-tracing effectively. “By now, what I was expecting is that 100% of people coming in contact with COVID-19 would have been traced,” says Nyenswah, now an infectious-diseases researcher at Johns Hopkins Bloomberg School of Public Health in Baltimore, Maryland.

Across the Western world, countries have floundered with this most basic public-health procedure. In England, tracers fail to get in touch with one in eight people who test positive for COVID-19; 18% of those who are reached provide no details for close contacts. In some regions of the United States, more than half of people who test positive provide no details of contacts when asked. These statistics come not from the first wave of COVID-19, but from November, long after initial lockdowns gave countries time to develop better contact-tracing systems.

The reasons for the failures are complex and systemic. Antiquated technology and underfunded health-care systems have proved ill-equipped to respond. Wealthy nations have struggled to hire enough contact-tracers, marshal them efficiently or make sure that people do self-isolate when infected or that they quarantine when a close contact has the disease. And overstretched contact-tracers have been met with distrust by people wary both of health authorities and of the technologies being deployed to fight the pandemic. Meanwhile, researchers who are keen to draw lessons from contact-tracing operations are stymied by a dearth of data.

A handful of places stand out as exemplars of successful contact-tracing — including South Korea, Vietnam, Japan and Taiwan. Many of these have cracked down on COVID-19 early, isolated infected people and their contacts and used personal data such as mobile-phone signals to track obedience. Not all of those techniques are transferable to countries now struggling to contain massive outbreaks. But they still provide some lessons.

Measures that work include tracing multiple layers of contacts, investigating outbreak clusters and providing people who are advised to quarantine with safe places to do so and with financial compensation. Technology might help, too: from software that streamlines conventional contact-tracing efforts, to smartphone apps that alert people that they might have been exposed to SARS-CoV-2.

A string of failures

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 day¹. 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.

Because of this series of problems, researchers estimate that in England this year, tracers typically reached less than half of the close contacts of people who’d had a positive COVID-19 test (see ‘Missed contacts’). There are no data on how many of these contacts actually quarantined in turn.