Canadians can now begin downloading a voluntary smartphone app meant to warn users if they’ve been near someone who tests positive for COVID-19.

Prime Minister Justin Trudeau says he downloaded the “COVID Alert” app Friday and says the more people who sign up to use it, the better it will be able to trace and help to slow the spread of the novel coronavirus.

The app is designed to track the location of phones relative to each other, without collecting personal data anywhere centrally, using digital identifications unique to each device.

Then users can be notified if their phones have recently been near the phone of a person who later volunteers that they have tested positive for COVID-19.

The app will then encourage users to call their provincial health services for advice on what to do next.

Trudeau says the app is currently linked to the Ontario health system, but anyone in Canada can begin using it Friday and more provinces are joining it soon.