The missing links: Evaluating contact tracing with incomplete data in large metropolitan areas during an epidemic

Contact tracing (CT) is a frontline measure against emerging epidemics, yet in practice it is never complete.

The quantitative impact of missing information -- such as untraced cases or unnotified contacts -- on the effectiveness of CT remains insufficiently understood.

Using a stochastic agent-based model with sociodemographics from metropolitan areas in South Korea, we simulate how different forms of information loss affect epidemic spreading dynamics.

We construct information-loss scenarios based on two types: infector-omission (IO), the omission of infected individuals from the tracing process, and contact-omission (CO), the omission of specific contact events even when the infected individuals themselves are identified.

The sensitivity of epidemic dynamics to increasing omission rates differs markedly between the two types: IO produces substantially stronger and more abrupt changes in transmission structure and epidemic outcomes, whereas CO produces more gradual effects.

Notably, CT effectiveness breaks down beyond a city-specific threshold -- an IO rate of approximately 4% in Seoul but about 10% in less populous Busan -- underscoring that CT strategies must be tailored to regional population and mobility structure.

Both IO and CO scenarios also lead to an increase in the transmission network diameter as information loss grows, indicating that a small network diameter reflects effective contact tracing that limits the depth of transmission chains.

Collectively, our results offer threshold estimates and practical guidance for designing robust CT systems in the real world.