Feature

WHERE COVID
CONTACT-TRACING
WENT WRONG
Many rich nations have struggled with a standard
technique for controlling disease. By Dyani Lewis

W
hen Ebola ripped through marshal them efficiently or make sure that
communities in West Africa people do self-isolate when infected or that
between 2014 and 2016, they quarantine when a close contact has the
Tolbert Nyenswah saw at first disease. And overstretched contact-tracers
hand how health workers have been met with distrust by people wary
extinguished the epidemic by both of health authorities and of the technol-
finding and quarantining con- ogies being deployed to fight the pandemic.
tacts of those who caught the Meanwhile, researchers who are keen to draw
disease. The former director of Liberia’s pub- lessons from contact-tracing operations are
lic-health institute thought contact-tracers stymied by a dearth of data.
would again rise to the challenge this year, A handful of places stand out as exemplars
keeping COVID-19 in check as it swept the of successful contact-tracing — including
globe. “Contact-tracing is one of the greatest South Korea, Vietnam, Japan and Taiwan. Many
tools that countries should deploy and use of these have cracked down on COVID-19 early,
effectively to contain the outbreak,” he says. isolated infected people and their contacts
But nine months after the World Health and used personal data such as mobile-phone might qualify. Tracers then call or visit those
Organization (WHO) labelled COVID-19 as a signals to track obedience. Not all of those contacts to tell them they need to quarantine,
pandemic, few countries are wielding con- techniques are transferable to countries now so that they don’t pass the virus on to more
tact-tracing effectively. “By now, what I was struggling to contain massive outbreaks. But people. The chain of transmission is broken.
expecting is that 100% of people coming in con- they still provide some lessons. In reality, failures occur at every stage of
tact with COVID-19 would have been traced,” Measures that work include tracing multiple this test–trace–isolate sequence. People get
says Nyenswah, now an infectious-diseases layers of contacts, investigating outbreak clus- COVID-19 and don’t know it, or delay getting
researcher at Johns Hopkins Bloomberg School ters and providing people who are advised to tested. Positive results can take days to be
of Public Health in Baltimore, Maryland. quarantine with safe places to do so and with confirmed. Not everyone who tests positive
Across the Western world, countries have financial compensation. Technology might isolates when requested; one survey in May
floundered with this most basic public-health help, too: from software that streamlines con- found that in the United Kingdom, 61% of
procedure. In England, tracers fail to get ventional contact-tracing efforts, to smart- people who were self-isolating said they’d
in touch with one in eight people who test phone apps that alert people that they might left their house in the past day1. People can’t
positive for COVID-19; 18% of those who are have been exposed to SARS-CoV-2. always be reached for an interview or don’t
reached provide no details for close contacts. provide details of their close contacts. And
In some regions of the United States, more A string of failures not all contacts are reached, or are willing to
than half of people who test positive provide The textbook version of contact-tracing starts comply with quarantine orders.
no details of contacts when asked. These statis- with someone testing positive for COVID-19 Because of this series of problems, research-
tics come not from the first wave of COVID-19, and isolating themselves. A contact-tracer ers estimate that in England this year, tracers
but from November, long after initial lock- interviews this person to find out who they typically reached less than half of the close
downs gave countries time to develop better might have exposed while infected, usually contacts of people who’d had a positive
contact-tracing systems. from 48 hours before the positive test, or COVID-19 test (see ‘Missed contacts’). There
The reasons for the failures are complex and before symptoms appeared (if there were are no data on how many of these contacts
systemic. Antiquated technology and under- any). Close contacts — those who’ve spent actually quarantined in turn.
funded health-care systems have proved ill- more than 15 minutes close to the infected The United States is in a particularly dire
equipped to respond. Wealthy nations have person — are of special interest, but anyone situation. “Public-health authorities are strug-
struggled to hire enough contact-tracers, who shared public transport or an office space gling to reach cases and contacts” despite

384 | Nature | Vol 588 | 17 December 2020 | Corrected 17 December 2020

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 In one case, a person was wrongly accused of
having an affair with his sister-in-law because
their overlapping maps revealed they dined
together at a restaurant.
Tracers in Vietnam also use extra data —
such as Facebook or Instagram posts and
mobile-phone location data — to check a
person’s movements against those reported
to contact-tracers. But the country’s success
was down to “the boots on the ground”, says
Todd Pollack, an infectious-disease specialist
at the Partnership for Health Advancement
in Vietnam, a collaboration that provides
training and support for the nation’s health
system. Contact-tracers interview people
face-to-face and use the extra surveillance
data to prod for more details. Other places,
including Israel, Armenia, Russia, Ecuador and
Taiwan, gather mobile-phone location data to
aid contact-tracing efforts. But in Slovakia, a
constitutional court suspended the govern-
ment’s attempt to permit this practice.
Once tracers in Vietnam identify close
contacts, they send them to designated
quarantine facilities. It’s a practice that has
worked elsewhere — including in Taiwan. Since
February, the WHO has recommended this
approach for suspected and confirmed cases,
as well as for close contacts of cases, particu-
larly if an infected person is unable to isolate
themselves from others in their household4.
Hong Kong and South Africa have non-man-
datory quarantine facilities for close contacts,
and others have facilities for travellers from
abroad, but most countries lack out-of-home
quarantine. In the United States, it’s estimated
that one in five households lacks the space
required to keep others in the house safe5.
Health-care workers conduct contact-tracing amid the COVID-19 pandemic in Soyapango,
El Salvador, in July. Against the clock
The WHO’s benchmark for a successful
aggressive efforts, says John Oeltmann, comparison. This makes it difficult to compare COVID-19 contact-tracing operation is to trace
head of contact-tracing assessment at the US failures in contact-tracing between countries. and quarantine 80% of close contacts within
Centers for Disease Control and Prevention 3 days of a case being confirmed — a goal few
(CDC) in Atlanta, Georgia. He and his team Finding contacts countries achieve.
evaluated two counties in North Carolina. In South Korea, authorities use data-surveil- But even that’s not quick enough, says
In June and July, 48% of cases in one county lance techniques to get around the problem of Christophe Fraser, a mathematical biologist
and 35% in the other reported no contacts. people being unwilling to disclose — or unable at the University of Oxford, UK. Transmission
Of the contacts whose details were provided, to recall — close contacts3. “We need to dou- is too rapid and the virus can spread before
one-quarter in one county and almost half in ble-check,” says Daejoong Lee at the South symptoms emerge, he points out. Modelling
the other couldn’t be reached on the phone Korean Ministry of Economy and Finance. by Fraser and his team suggests that even if all
after three attempts over consecutive days2. In A law passed in response to an outbreak of cases isolate and all contacts are found and
New Jersey, just 49% of cases between July and Middle East respiratory syndrome (MERS) quarantined within three days, the epidemic
November were contacted; only 31% of those in 2015 allows authorities to use data from will continue to grow. He says that in a single
provided any contact details. “These results credit cards, mobile phones and closed-circuit day, 70% of cases need to isolate and 70% of
are not rare,” says Oeltmann. television to trace a person’s movements and contacts need to be traced and quarantined for
Such data, which demonstrate how poorly identify others they might have exposed to the the outbreak to slow (defined as each infected
contact-tracing is working, are scarce. Only virus. Information about cases is published person passing the virus to fewer than one
a handful of US states openly report con- online, an approach that allowed the coun- other, on average)6.
tact-tracing metrics. And although the try to avoid broad lockdowns and “worked But there are ways that contact-tracers can
JOSE CABEZAS/REUTERS

European Centre for Disease Prevention very well”, says Lee. Still, in March, the Korea get ahead of a rapidly spreading outbreak.
and Control (ECDC) lists the types of data Centers for Disease Control issued guidelines One is to cast a wider net around each case,
that countries should gather to monitor limiting the release of ‘excessive’ information, so that second-order contacts — ‘contacts of
contact-tracing efforts, none is reported after regional governments published maps of contacts’ — are traced and quarantined; in
back to the ECDC or is readily available for infected people’s routes in too much detail. Vietnam, tracers sometimes reached out to

Corrected 17 December 2020 | Nature | Vol 588 | 17 December 2020 | 385

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Feature
third-order contacts if a case was identified countries had such people in reserve when Washington DC, says that a national or regional
late in its infectious cycle. As many as 200 COVID-19 hit, perhaps because outbreaks of workforce could service different commu-
contacts for each case are found and tested, infectious diseases are less common in those nities as hotspots occur. Private call-centre
says Pham Quang Thai, an epidemiologist regions. In April, a US report (see go.nature. contractors are another workaround, but
at the National Institute of Hygiene and Epi- com/348knvz) estimated that around 30 con- this strategy has mixed results. Outsourcing
demiology in Hanoi, who leads the national tact-tracers per 100,000 people are needed worked well in Massachusetts, says Marcus
contact-tracing taskforce. “If we want to run during surges in case numbers — a nationwide Plescia, the chief medical officer at the US
as fast as the virus, we have to chase not only workforce of nearly 100,000. At the time, the non-profit Association of State and Terri-
the first round,” he says. United States had only 2,200 contact-tracers. torial Health Officials, in Charlotte, North
“Tracing the contacts of contacts is a great It now has around 50,000, but this ranges from Carolina. But in England, poor coordination
strategy,” says Nyenswah. But in many places, 60 per 100,000 in Washington DC to 2 per between contractors and health authorities
he says, even tracing the first ring of contacts 100,000 in Montana and Iowa. Vietnam had left call-centre staff underused despite huge
is proving tough. more than 12,000 trained contact-tracers at demand, according to media reports. Selina
The number of contacts identified for each the outset. Rajan, a public-health specialist at the London
COVID-19 case varies wildly, from an average of Many regions have bolstered tracing efforts School of Hygiene & Tropical Medicine, says
17 per case in Taiwan, to 2 in the United King- using students, off-work airline staff and vol- that the quality of information also suffered.
dom, 1.4 in France and less than one in parts “It’s focused very much on the supply of people
of the United States. During the outbreak of “If there’s a single lesson to make calls and not necessarily on the out-
severe acute respiratory syndrome (SARS) in come,” she says. “I don’t actually think those
Toronto in 2003, around 23,000 contacts for
that every country needs people are really qualified enough.”
an estimated 251 cases — nearly 100 contacts to learn, it’s invest in your In the United States, public-health officials
per case — were told to quarantine. But it’s diffi- public–health system.” are faced with tough choices as tracers strug-
cult to interpret these numbers. A country with gle to keep up. Some are prioritizing at-risk
no restrictions on movement will inevitably groups, or are ceasing to check on people asked
need to trace more contacts than one that is in unteers, but have still struggled as case num- to quarantine. The CDC now recommends that
some form of lockdown, for instance. bers rise. Germany planned to recruit to the tracers focus on cases infected in the past six
Another useful strategy is to trace a new level of 25 per 100,000, but imposed a second days, and encourages cases to notify their own
case’s contacts as far back as a fortnight lockdown. So, too, did England, which has a household contacts. In California and New
before they caught the virus, to identify who tracing workforce close to 32 per 100,000. In Mexico, lockdowns have returned.
infected them. This ‘backwards contact trac- countries that got on top of their outbreaks Large swathes of the world now have wide-
ing’ is extremely effective for the coronavirus quickly, that level of workforce expansion was spread community transmission, meaning
because of its propensity to be passed on in never required. Taiwan’s population of 24 mil- that numerous cases can’t be linked to identi-
superspreading events, says Adam Kucharski, lion was served by 600 contact-tracers at the fied ones — a sign that contact-tracing is fail-
an infectious-diseases modeller at the London peak of its outbreak — just 2.5 contact-tracers ing to keep pace. In Vietnam, by contrast, “less
School of Hygiene & Tropical Medicine. One per 100,000 (ref. 8). than 1%” of cases during the latest outbreak
study in Hong Kong found that 19% of cases of Edward Salsberg, a health-workforce econ- had an unknown source, says Thai. These cases
COVID-19 were responsible for 80% of trans- omist at George Washington University in prompted a swift response, usually in the form
mission, and 69% of cases didn’t transmit the of a hyper-local lockdown — of a single street,
virus to anyone7. (The SARS outbreak in 2003 MISSED CONTACTS village or suburb — to contain the outbreak.
had similar transmission dynamics.) Any new Between late May and November, contact-tracers in Vietnam “just did everything you’re supposed
case is more likely to have emerged from a clus- England generally reached less than half of the close to do”, says Pollack. Responders acted more
contacts of people who tested positive for COVID-19.
ter of infections than from one individual, so quickly and maybe more comprehensively,
there’s value in going backwards to find out Confirmed cases of COVID-19 reached he adds, “but it’s not like they did something
who else was linked to that cluster. Japan rec- by contact-tracers magical”.
ognized this feature early and adopted clus- Confirmed cases who gave details of Still, high case numbers aren’t an excuse
their close contacts
ter-focused contact-tracing in February; it to give up on contact-tracing, Rajan says. UK
Estimated proportion of contacts
traces contacts up to 14 days before symptom whom tracers reached government science advisers have been criti-
onset, rather than the usual 48 hours. * cized for saying in February that the practice
In Australia, contact-tracing lessons have 100 “should be discontinued” when cases got
been learnt the hard way. A second wave too high — advice the government followed
of COVID-19 cases sent the country’s sec- in March — rather than urging the country
80
Contact-tracing success (%)

ond-largest city of Melbourne into a marathon to build up its tracing capacity. Abandoning
lockdown in August that lasted for 112 days. By contact-tracing left epidemiologists blind to
the time restrictions eased at the end of Octo- 60 details about where and why new cases were
ber, contact-tracers had adopted the Japanese cropping up, Rajan says.
practice of backwards-tracing contacts for the 40
previous 14 days, and the Vietnamese prac- Technology tricks — and troubles
SOURCE: HTTPS://COVID.I-SENSE.ORG.UK

tice of quarantining first- and second-order Smart data-management systems can ease
contacts. The overhauled system is yet to be 20 the workload of contact-tracers and help
tested, because the lockdown eliminated com- countries get by with fewer disease detec-
munity transmission. 0 tives. Many nations, including South Korea,
28 May 2 Jul 6 Aug 10 Sep 15 Oct 19 Nov Vietnam and Germany, developed their own;
An army of tracers Week start state governments in Australia and the United
All of these approaches require an army of States are adopting a commercial system.
*Number rises over 100% because some positive cases
contact-tracers, but few affluent Western were carried over from the previous week. These are useful because they can record who

386 | Nature | Vol 588 | 17 December 2020

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 concerns about contact-tracing,” she says.
“Public trust in all sorts of institutions is
declining,” says sociologist Robert Groves,
former director of the US Census Bureau,
who notes that this is especially the case in
large urban areas where social cohesion has
also declined. But the low numbers of people
providing details of contacts or responding to
calls from contact-tracers, while disappoint-
ing, are not surprising, says Mary Bassett, a
public-health researcher at Harvard University
in Cambridge, Massachusetts. Some commu-
nities that have been hardest hit by COVID-19
have a long-standing distrust of public-health
authorities, she says. “For the African American
NICOLAS ASFOURI/AFP/GETTY

community, there’s a history of malfeasance

on the part of the public-health system,” she
says, “and for the Latino community, there’s a
problem of members of the community who
are undocumented” — and fear deportation.
Systems are often hampered by a lack of
Beijing controls residents’ movements with smartphone scanners to quell outbreaks. support for people who fall ill or need to quar-
antine, too. Providing adequate financial com-
has been contacted and avoid repeated phon- could notify contacts almost instantaneously. pensation for personal hardship as a result of
ing of people who are contacts of multiple Apps can also help shop owners and restau- quarantine could shift people’s reluctance to
cases, says Karin Verspoor, a computational rants to log who has visited their premises. comply. The prospect of being without income
linguistics researcher at the University of Mel- Contact-tracing apps have met with con- for two weeks — or losing a job entirely — is a
bourne — a situation that has been a problem cerns over privacy, and although they are big burden, says Plescia, and might explain
in the United Kingdom and Australia. becoming common — at least 46 countries people’s reluctance to provide details for their
Other tasks typically managed by con- have developed some form of app — uptake close contacts.
tact-tracers can also be delegated to tech- rates remain low. “Adoption is not phenomenal If Western countries are not prepared to
nology. In Vietnam, contacts log their health anywhere,” says Effy Vayena, a bioethicist at enforce case isolation (as Singapore does),
status through a symptom-tracking app, the Swiss Federal Institute of Technology in quarantine contacts separately (as happens in
freeing up contact-tracers to interview newly Zurich, who is involved in the development Vietnam) or adopt digital measures to identify
infected individuals. People can also report of the Swiss contact-tracing app. “The trust people (as in South Korea), some might ask
the movements or symptoms of relatives who question is crucial,” she says. It is exacerbated whether contact-tracing is up to the task of
might not own a smartphone. In South Korea, by a history of large-scale data breaches and suppressing the spread of COVID-19 in these
Kenya and South Africa, authorities use phone privacy scandals in digital technologies. locations — even if nations get better at recur-
location data to identify quarantine breaches. Despite the low uptake, Fraser says, apps sive and backwards tracing .
At the beginning of the pandemic, over- are a worthwhile adjunct to manual efforts, But Nyenswah isn’t writing it off. Even in
stretched contact-tracers in the United States, because small effects can accumulate over the United States, he says, the practice could
Australia and the United Kingdom faced the time. “This isn’t a silver bullet,” he says, but yet work to bring case numbers down. It will
extra burden of antiquated health-care sys- “it’s not just a gimmick”, either. be difficult, but it’s essential for regional and
tems. In Australia, as well as in US states such as national leaders to clearly communicate the
Hawaii and Washington, health departments Trust deficit importance of contact-tracing, he says. “There
are often notified of new cases by fax or phone. For contact-tracing to work, people with is no substitute for political leadership in an
“It’s somewhat embarrassing,” says Plescia, COVID-19 must be prepared to answer ques- outbreak response.”
but “we never invested in the systems to allow tions about their whereabouts, and they must
them to do it differently”. Entering names and isolate themselves from others while unwell. Dyani Lewis is a freelance science journalist in
other details into a database from faxed notifi- In many places, that’s not happening. Melbourne, Australia.
cations causes big delays, he says, so that the A survey of attitudes to contact-tracing
1. Smith, L. E. et al. Public Health 187, 41–52 (2020).
window during which contact-tracing might across 19 countries in August found that 2. Lash, R. R. et al. Morb. Mortal. Wkly Rep. 69, 1360–1363
make a difference vanishes. nearly three-quarters of respondents would (2020).
“If there’s a single lesson that every country be willing to provide contact information9. 3. COVID-19 National Emergency Response Center.
Osong Public Health Res. Perspect. 11, 60–63 (2020).
needs to learn, it’s invest in your public-health But rates varied. In Vietnam, only 4% of par- 4. World Health Organization. Considerations for Quarantine
system,” says Rajan. ticipants said that they wouldn’t provide this of Individuals in the Context of Containment for
information. In the United States and Ger- Coronavirus Disease: Interim Guidance, 29 February 2020
(WHO, 2020).
Assistance from apps many, the proportion was 21%, and in France, 5. Sehgal, A. R., Himmelstein, D. U. & Woolhandler, S.
One idea touted early on was to do con- it was 25%. Concerns around data privacy and Ann. Intern. Med. https://doi.org/10.7326/M20-4331
tact-tracing with smartphone apps. These tracking are partly to blame, says researcher (2020).
6. Ferretti, L. et al. Science 368, eabb6936 (2020).
emit Bluetooth signals to other phones; when Sarah Jones at Imperial College London, who 7. Adam, D. C. et al. Nature Med. 26, 1714–1719 (2020).
a person tests positive for coronavirus, their co-led the survey. “Many health authorities 8. Jian, S.-W., Cheng, H.-Y., Huang, X.-T. & Liu, D.-P. Int. J.
phone app notifies others who were in close and governments, especially in North Amer- Infect. Dis. 101, 348–352 (2020).
9. Institute of Global Health Innovation. COVID-19:
proximity for 15 minutes or more (if they have ica and Western Europe, may need to urgently Perceptions of Contact Tracing. Global Report (IGHI,
installed the app). In theory, such technology improve public-health messaging to mitigate 2020).

Nature | Vol 588 | 17 December 2020 | 387

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 Correction
This feature referred to the wrong guidance
from the WHO on quarantining contacts in
designated facilities. It should have referred
to guidance issued in February (World Health
Organization. Considerations for Quarantine
of Individuals in the Context of Containment
for Coronavirus Disease: Interim Guidance,
29 February 2020; WHO, 2020), not that
issued in August. Furthermore, it erred in
stating that the WHO declared COVID-19 a
pandemic. In fact, it characterized the out-
break as having reached pandemic levels.

388 | Nature | Vol 588 | 17 December 2020

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