On 6 April, we approached the Belgian government with concerns about the improper use of contact-tracing smartphone apps for controlling pandemics. These concerns were in line with those you discuss (Nature 580, 563; 2020). On 17 April, we drew its attention to other issues relating to lockdown exit strategies.

We argued that contact-tracing apps could complicate rather than facilitate lockdown exit (see go.nature.com/36ebfmq). For example, receiving (or not) a warning through the app might elicit a false sense of security, or drive demand for testing that might not be available. And there is more at stake than the government’s public-health-efforts and investment: an app’s success also depends on personal, public and social trust.

Governments need to engage stakeholders to co-design the app so that it aligns with local culture and connects with vulnerable populations. They also need to use proper information campaigns and human follow-up after issuing app warnings, and to ensure that the media accurately relay what the apps can and cannot deliver.

For now, the Belgian government has paused its implementation of contact-tracing apps (see go.nature.com/2zinmbb). If they pursue the project, we hope it will incorporate the necessary caution and guidance for citizens.