Study Of Chinese City Shows Contact Tracing, Isolation Key To Mitigating Spread Of Coronavirus

SHENZHEN, China — A new in-depth analysis of containment measures taken in the city of Shenzhen, China during the early outbreak of SARS-CoV2 has concluded that contact tracing and subsequent isolation measures were incredibly effective at slowing the spread of coronavirus. Contact tracing refers to quickly identifying individuals who likely came into contact with SARS-CoV2 and placing them under isolated quarantine.

A total of 391 confirmed coronavirus cases and 1,286 of their close contacts were investigated for this research. According to the study’s calculations, contact tracing measures taken in Shenzhen sped up new case detection by two full days and reduced the time it took to place possibly infected individuals under quarantine by another two days. All in all, these measure greatly reduced the number of infections, as well as the amount of time locals were infectious in the community, over a four week time period (1/14-2/12) earlier this year.

“The experience of COVID-19 in the city of Shenzhen may demonstrate the huge scale of testing and contact tracing that’s needed to reduce the virus spreading,” says Dr Ting Ma from the Harbin Institute of Technology at Shenzhen, China, in a release. “Some of the strict control measures enforced here, such as isolating people outside their homes, might be unlikely to be replicated elsewhere, but we urge governments to consider our findings in the global response to COVID-19. To achieve similar results, other countries might be able to combine near-universal testing and intensive contact tracing with social distancing and partial lockdowns. Although no lockdown measures were introduced in Shenzhen until the end of our study period, Wuhan’s lockdown could have significantly restricted the spread of coronavirus to Shenzhen.”

Regarding the 1,286 close contacts of confirmed COVID-19 cases, even people who didn’t show any symptoms were placed under quarantine. This ensured that asymptomatic carriers weren’t unknowingly spreading the virus. A “close contact” was defined as someone who lived with, ate with, or traveled with an infected person for up to two days before that patient started showing symptoms

Normally, it took an average of 4.6 days for a local to be isolated due to COVID-19 symptoms. With contact tracing, that average dropped to 2.7 days. A total of 87 studied people who were contact traced and tested ended up being positive for the coronavirus, a fifth of those people didn’t have any symptoms and 30% didn’t have a fever. So, contact tracing stopped those individuals from unknowingly spreading the virus.


Contact tracing also reduced the time lag between a person developing symptoms and being properly diagnosed. Without contact tracing there was a 5.5 day delay, but with contact tracing that dropped to only 3.2 days.

Interestingly, only 11% of studied close contacts who shared a living space with a confirmed patient actually tested positive themselves. Similarly, 6% of close contacts who shared a meal with a confirmed patient tested positive, as well as 11% of close contacts who traveled with a COVID-19 patient. However, the study’s authors believe transmission rates are likely higher in other countries with less strict containment measures.

Of course, it’s impossible to track every single person another individual comes into contact with, which is why close contacts were primarily focused on.

“As we look towards post-lockdown strategies, we should examine the experience of countries that have successfully controlled SARS-CoV-2 transmission or have low mortality (eg, China, Singapore, Taiwan, South Korea, Germany, and Iceland). Successful strategies include ample testing and contact tracing, supplemented by moderate forms of social distancing,” comments Dr Cécile Viboud from the National Institutes of Health, USA.

Contact tracing on the scale that is needed for the SARS-CoV-2 response is labour intensive, and imperfect if done manually. Hence new technology-based approaches are greatly needed to assist in identification of contacts, especially if case detection is aggressive. Building on the SARS-CoV-2 experience in Shenzhen and other settings, we contend that enhanced case finding and contact tracing should be part of the long-term response to this pandemic–this can get us most of the way towards control,” Dr. Viboud concludes.

The study is published in The Lancet Infectious Diseases.

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