SYDNEY Australia unveiled a dramatic initiative to trace new cases of the coronavirus within its borders this week: An app government officials hope 40% of the population will soon download.

The government hopes its effort, called COVIDsafe , will help officials effectively trace many of the contacts a person infected with the virus may have had while they were contagious, a step deemed essential by health professionals to contain any new outbreak.

Users are asked to provide a name (or pseudonym), age range, area code and phone number. Each time a phone comes within 5 feet from another device with the app, the two share a digital, encrypted handshake. If someone using the app later tests positive for the virus, the anonymized handshakes from the previous 14 days are uploaded with permission and then used for contact tracing. Those that were near the infected person can then be notified and tested themselves.

Officials have said just under half the population would need to download it to make it a success. Prime Minister Scott Morrison, who has stressed downloads will not be mandatory, has likened the effort to citizens buying war bonds in an appeal to an Australia homing in the potential elimination of local cases .

I will be calling on Australians to do it, frankly, as a matter of national service, to do it in the same way that people used to buy war bonds back in the war times, to come together to support the effort, he said earlier this month. I know this would be something they might not normally do in an ordinary time, but this is not an ordinary time.