Some apps, like the one built by Singapore, constantly broadcast Bluetooth pings to find nearby devices. Others, like the one built by the UK, try to create active Bluetooth pairings or “handshakes.”

Already, the UK government appears to have undermined prior assurances that it won’t share the data it collects outside the NHS, suggesting other organizations might use the information for public health research in the future. This is something Apple and Google forbid for any app using their API, and another reason the UK has to build its app without the companies’ help.

The UK is one of the few countries that has chosen to create a contact-tracing app that is incompatible with the contact-tracing API currently being developed by Google and Apple. Instead of decentralizing the data across devices, the UK will pool the information it collects in a single database operated by the National Health Service, or NHS.