[BostonCard]

Since it is hard to know exactly when someone was exposed and there is a fair bit of uncertainty around the incubation period, when you contact trace close contacts you are testing both prospective and retrospective contacts simultaneously, as long as you use a broad enough time period.  Retrospective contact tracing is harder, because you generally have to trace further back in time, and memories fade.

So you are right, you are mostly going to catch prospective contacts.  But that doesn't mean you don't try.  It's not about having to be perfect, but the more transmission chains you intercept, the further you drive down R, and the faster you drive down the burden of disease.

BC

[BostonCard]

Since it is hard to know exactly when someone was exposed and there is a fair bit of uncertainty around the incubation period, when you contact trace close contacts you are testing both prospective and retrospective contacts simultaneously, as long as you use a broad enough time period.  Retrospective contact tracing is harder, because you generally have to trace further back in time, and memories fade.

So you are right, you are mostly going to catch prospective contacts.  But that doesn't mean you don't try.  It's not about having to be perfect, but the more transmission chains you intercept, the further you drive down R, and the faster you drive down the burden of disease.

BC