A translator who jumps at every rustle in the hall turns a one-hour meeting into two. It's the same in epidemic control: contact tracing teams try to reconstruct the chain of infections, but something always slips through. Someone forgets who they met, someone doesn't answer the phone, someone never even comes to the attention of health workers. The question is how critical these gaps are.
Researchers built a computer model based on data from residents of two major South Korean cities — Seoul and Busan. In the model, virtual people go to work, ride public transit, meet each other, and the virus spreads through these contacts. Then the scientists deliberately "blinded" the tracing system: in one scenario, they missed a fraction of infected individuals; in another, a fraction of specific encounters. They watched how the epidemic picture changed.
The difference was striking. When infected individuals are missed, the epidemic flares up sharply and violently: transmission chains break in the most unexpected places, and the virus gains freedom. When individual encounters are missed, the effect builds gradually, like water dripping from a faucet. But the most interesting part is the threshold beyond which tracing stops working. In Seoul, losing sight of just 4% of infected individuals is enough to collapse the system. In Busan, it's about 10%. Why? Because the cities are structured differently: in a megacity, people have more frequent and denser contacts, and every missed thread pulls a whole tangle behind it.
Another finding: as missed cases increase, the "diameter" of the transmission network grows — the largest number of handshakes between any two infected individuals. A short diameter means the virus doesn't get far from the first case. When tracing works well, chains are short and easier to break. When information is lost, the virus carves long, winding paths, making it nearly impossible to catch.
This work isn't about abstract numbers. It provides a practical benchmark: health services need to know their "blindness threshold" and design their system to stay well below it. For Seoul, that means near-perfect handling of every infected person; for Busan, a bit more margin for error. In any case, the cost of a mistake isn't just one missed encounter: it's an entire wave of new cases.
In South Korea, contact tracing relies on data from bank cards, phones, and surveillance cameras, allowing the movement of an infected person to be reconstructed to the minute.
In the film Contagion (2011), epidemiologists work to reconstruct the contact chain of the first infected person, and every mistake costs more lives.