  Tracing potentially infected contacts of confirmed cases is important when
fighting outbreaks of many infectious diseases. The COVID-19 pandemic has
motivated researchers to examine how different contact tracing strategies
compare in terms of effectiveness (ability to mitigate infections) and cost
efficiency (number of prevented infections per isolation). Two important
strategies are so-called forward contact tracing (tracing to whom disease
spreads) and backward contact tracing (tracing from whom disease spreads).
Recently, Kojaku and colleagues reported that backward contact tracing was
``profoundly more effective'' than forward contact tracing, that contact
tracing effectiveness ``hinges on reaching the `source' of infection'', and
that contact tracing outperformed case isolation in terms of cost efficiency.
Here we show that these conclusions are not true in general. They were based in
part on simulations that vastly overestimated the effectiveness and efficiency
of contact tracing. Our results show that the efficiency of contact tracing
strategies is highly contextual; faced with a disease outbreak, the disease
dynamics determine whether tracing infection sources or new cases is more
impactful. Our results also demonstrate the importance of simulating disease
spread and mitigation measures in parallel rather than sequentially.
