Background
The COVID-19 pandemic highlighted the growing challenge of presymptomatic transmissions. This study synthesises evidence on the role and effectiveness of informal contact tracing (ICT) - a decentralised, problem-oriented approach where infected individuals notify close contacts, enabling earlier awareness and prevention of presymptomatic transmissions.

Methods
Following PRISMA guidelines, we systematically reviewed the literature across eight online databases. We also developed a model to estimate the effectiveness of ICT.

Results
The review highlighted four studies showcasing ICT benefits, including 1) scalability and speed: ICT enables rapid notification without centralised infrastructures; 2) early intervention: notified contacts gain awareness of potential infection 1 - 2 days earlier than formal tracing, reducing secondary spread and enabling timely antiviral treatment; 3) behavioural influence: encourages precautionary behaviours among asymptomatic carriers; 4) community resilience: fosters trust, solidarity, and mutual accountability. Modeling demonstrated ICT's capacity to disrupt short-generation transmissions, reducing effective reproduction number (from 1.31 to 0.80) in scenarios with realistically achievable public adherence. Over a 30-day period beginning on 6 June 2022, the baseline public health intervention model scenario prevented 971,957 COVID-19 cases. The deployment of promoted ICT in conjunction with vaccination programs has the potential to mitigate presymptomatic spread.

Conclusions
Despite empirical data on efficacy are limited, theoretical and modeling evidence supports ICT as a transformative, community-driven strategy for pandemic management. ICT's ability to target undetected transmission, empower individuals, and reduce dependence on restrictive measures aligns with key public health priorities. Policymakers should integrate ICT into public health frameworks as an effective complement to existing contact tracing approaches.

Key messages
The developed model shows that optimizing key performance parameters (e.g., detection and notification performance, minimal delays, protective measures) through ICT can curb an epidemic's growth phase.

ICT shows promise as an effective strategy, especially when integrated with broader public health efforts to address early transmission pathways during infectious disease outbreaks.