Extensive contact tracing, isolation and testing key to control Covid-19 spread, says Lancet study

BEIJING: Extensive contract tracing, isolating cases, and testing may reduce the spread of the novel coronavirus, and favour control of the COVID-19 outbreak, suggests a study carried out in China.

However, the researchers from the Harbin Institute of Technology at Shenzhen, China, caution that the impact of contact tracing to rapidly isolate people who could be infected with coronavirus disease 2019 (COVID-19) depends on identifying asymptomatic cases.

Published in The Lancet Infectious Diseases journal, the study of 391 cases and 1,286 of their close contacts found that contact tracing reduced the length of time people were infectious in the community over 4 weeks in Shenzhen, China.


According to results from In Shenzhen, authorities identified whom to isolate based on their contact with confirmed cases, as well as isolating people who already had symptoms, the researchers said.

The new study found that contact tracing increased the speed at which new cases were confirmed by 2 days - an average of 5.5 days initially, reduced to 3.2 with contact tracing.

It also reduced the amount of time it took to isolate infected people by 2 days - from an average of 4.6 days down to 2.7.

There were only three deaths in the study group during the study period, the researchers said.

"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," said Ting Ma from the Harbin Institute of Technology at Shenzhe

"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," said Ma.

To achieve similar results, the researchers noted that 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," Ma said.

For the current study, the researchers analysed data from 391 people diagnosed with COVID-19 after they showed symptoms, and 1,286 of their close contacts.

The contacts were tested irrespective of whether they had symptoms in order to identify infected people who were asymptomatic.

The data gave insights into the type of contact most likely to lead to transmission.

Close contacts were defined as people who shared a household with infected patients up to 2 days before they started showing symptoms, or interacted with them socially by travelling or eating together.

For people who were isolated because they showed symptoms of COVID-19, it took an average of 4.6 days for them to be isolated following the first signs of infection. Contact tracing reduced this to an average of 2.7 days, the researchers found.

For people diagnosed with COVID-19 after being contact traced and tested (87 people), a fifth (17 out of 87 people) had not yet developed any symptoms, and 30 per cent (25 out of 87) did not have a fever, they said.

The length of time for which a person remains infectious is not yet known, but reducing the amount of time that infected people interacted with others appears to have helped reduce the virus spreading, the researchers said.

The researchers highlight several limitations to their study, including that it is impossible to trace every potential contact an individual has.

Contact tracing therefore focuses on close contacts who are most likely to be infected.

They note that some infected travellers to Shenzhen could have been missed if they were only tested due to symptoms such as a fever.

Their contacts might also have been missed if they were asymptomatic, because the PCR test is not sensitive enough to pick up every case, according to the researchers.