South Korea, a nation of 51 million people, stands out as one of the most successful countries in the world in managing to contain Covid-19.

Much of this success has been attributed to the countrys ability to mobilise a small army of contact tracers: detectives trained to connect the dots between a positive Covid-19 case and all their most recent contacts. The tracers must then decide who should be instructed to self-isolate, or, in some cases, whether to quarantine an entire building or organisation, such as a hospital, care home or office.

New daily cases of Covid-19 in South Korea

New daily cases of Covid-19 in South Korea

With only a handful of cases throughout January and early February, many South Koreans thought perhaps a large outbreak had been avoided. However, in late February, from a single city, came a sudden escalation of thousands of cases reported within the space of only a few days.

The outbreak in Daegu city has since been attributed to the movements of one single person, South Koreas superspreader - the now infamous patient 31.

The city of Daegu accounts for more than 60% of all of the country's cases

The city of Daegu accounts for more than 60% of all of the country's cases

Patient 31 tested positive for coronavirus on 17 February. It was only thanks to the work of contract tracers, that all her most recent contacts - which, shockingly, turned out to be more than 1,000 people in the space of 10 days - were tracked down and instructed to self-isolate, thus avoiding an even bigger outbreak.

As deputy of Daegus epidemiology team, Professor Kim Jong-Yeon is responsible for the citys infantry of contact tracers - often former government employees, as well as junior doctors. He says only if people are evasive, do they use more rigorous methods - such as investigating their credit card transactions and their phone or GPS history. People such as patient 31.

Professor Kim Jong-Yeon in Daegu City

Professor Kim Jong-Yeon in Daegu City

Patient 31, at first she didn't tell us she was from the Shincheonji Church. It was us, the contact tracers, who later discovered she was a member, says Prof Kim.

With approximately 300,000 members nationwide, the doctrine of Shincheonji Church of Jesus claims their founder, Lee Man-hee, is the second coming of Jesus Christ and that only he can interpret the Bible. Many mainstream Christian churches in South Korea consider the group to be a cult and have long criticised their aggressive recruitment of young people.

A service at Shincheonji Gyeonggi-doChurch. Source: Shincheonji

A service at Shincheonji Gyeonggi-doChurch. Source: Shincheonji

But patient 31 has not become infamous simply for covering up her affiliation with Shincheonji Church. As contact tracers uncovered, in the 10 days prior to being tested - despite showing symptoms - she travelled around the city of Daegu coming into contact with more than 1,000 people.

After being involved in a car accident on 6 February, patient 31 was admitted to hospital on 7 February, where she came into close contact with an estimated 128 people. She then temporarily discharged herself in order to return home to collect personal belongings, a two-and-a-half-hour round-trip, before returning to hospital. Later that week she discharged herself multiple times, once going for lunch with a friend, and twice in order to attend a a two-hour church service with a 1000-strong congregation.

Due to the secretive nature of the Shincheonji Church, Prof Kim says the hardest part of the investigation was trying to establish who also visited the church during that week.

We finally secured a list of all 9,000 members of the church. At first, we started to call and ask all of them if they had any symptoms. About 1200 people told us they did, but some people refused to get tested and self-quarantine.

With hundreds of individuals reluctant to reveal their association with the secretive church, the professor says they were left with no choice.

It became a matter of how quickly we could separate those church members from the rest of Daegus citizens. So the government issued an executive order for all church members to self-isolate.

Thousands of students of the Shincheonji Church celebrate their in-house graduation. Source: Shincheonji

Thousands of students of the Shincheonji Church celebrate their in-house graduation. Source: Shincheonji

The citys rigorous investigation of all new cases, combined with comprehensive testing, quickly curbed the spread of the virus - and by early April, the city of Daegu reported zero new cases of Covid-19.

However, elsewhere in the world, the virus continued to advance unabated. For the scientific community, it became vital to track the virus, not only over borders, but across continents.

The answer to this problem lay in the genome, clues left behind in the genetic code of the virus as it began to replicate and spread.

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