This is the type of thing they need to be doing but if they don't couple it with a wider test program it may be pointless. How can they identify coronavirus carriers if they are doing such limited testing.
On the privacy aspect I did see a cryptographer comment recently about using privacy preserving crypto techniques to help hide keep data private. I will look to see if I can see anything on this.
This could be really helpful, didn’t they do something similar in China and South Korea? The problem I can see is with people not engaging with the app because of data protection concerns and privacy concerns, differences in way public views this level of interference and tracking and information gathering, different views of community, etc which is talked about in the article.
And I don’t think this app can be relied on, just by itself. Because say 20% of the infected people didn’t engage with the app, or even 10 or 5%, that’s probably enough just to cause a major outbreak and spread to so many people. If just one person didn’t cooperate even. So everyone would need to cooperate. Also how will the app by itself trace everyone? Wouldn’t other things be needed too, as in below article, they used camera footage, card details etc in S Korea. In China mobile phones were tracked extensively - one man who broke quarantine was called immediately by police and ordered to go back home within the hour.
In South Korea I’ve also read that the public have a high level of trust in their government, there is also a level of “community spirit” and also it’s enshrined in their laws that when it comes to matters of public health (like epidemics) these sort of things will need to be done.
I think this sort of app and tracking could be helpful but definitely not enough by itself.
“It was during this traumatic time that the South Korean public came to accept the costs to their privacy in return for the gains in government transparency — and thus, public safety. The revisions to public health law in the aftermath of MERS
reflectthis compromise. Today, under the amended Infectious Disease Control and Prevention Act (IDCPA), the minister of health exercises expansive power to collect private data of confirmed and potential patients. At the same time, the law grants the public a “right to know,” requiring the minister to “promptly disclose information” — including the movement paths, transportation means, and contacts of patients — to the public. This bargain was crucial to legitimizing the government’s track-and-trace strategy and mobilizing the public’s cooperation in their fight against COVID-19.
Testing was followed by extensive tracking and tracing. Once a case was confirmed, the authorities tracked down the movement histories of the patient and traced the people they had contacted. The authorities worked with local governments to
survey security camera footage, smartphone data, and credit card records to chart — down to the minute — the patients’ previous travels and contacts. The government also mandated and encouraged innovative ways of sharing this information. It used a GPS-tracking app to
oversee and publicize patients’ movements in real time and penalize those that broke quarantine. Further, it invited companies to develop apps that visualized the patients’ anonymized location data and made them more accessible to the public. One such app — called the “Corona 100m” —
alerted users when they came within 100 meters of the recent whereabouts of a coronavirus patient.
Simultaneously, the patients were
categorized by risk — asymptomatic, mild, severe, or critical — and treated accordingly. Higher-risk patients, including the elderly and seriously ill, were hospitalized. By contrast, lower-risk patients, such as the young and those showing moderate to no symptoms, were sent to dormitories borrowed from companies like Samsung and LG.”
https://thediplomat.com/2020/03/a-democratic-response-to-coronavirus-lessons-from-south-korea/
I'm sure there's a sensible explanation but it escapes me. 
