From my perspective, young adults tend to live in an urban area than other age groups because of study or work. It contributes to the usage of contact tracing apps through public awareness. Secondly, it probably finding more contacts in young adults in urbanized areas for contracting infection due to population density. “Urbanization” is associated with the exposure we have studied, and it is not an intermediate step in the causal path between exposure and outcome. It is also a risk of having more contacts in the study. Thus, I think Urbanization is one of the confounders, which is a causal relationship with contact patterns.