The EU's eHealth Network published a guideline for the interoperability framework for contact tracing apps, which EU member states have launched or are planning to launch to combat the Coronavirus. The guideline was developed as the first follow-up action to the EU Toolbox of mobile contact tracing application. The network emphasised that interoperability between apps and between back end systems is key for tracing cross-border infection chains. It calls on member states to ensure that apps are able to exchange only the minimum information necessary to notify users located anywhere in the EU of their contact with another user who has reported that they have contracted the Coronavirus.