According to the research Ars Technica referenced, digital
  contact tracing doesn't start becoming effective at 60% - 
  at 60% usage, digital contact tracing can stop the pandemic

There's no silver bullet with this virus. Contact tracing has proven ineffective in Singapore [1], even with additional layers of surveillance and thousands of dedicated staff reviewing CCTV footage, cell tower location data, and calling those suspected exposed to confirmed cases.

https://www.economist.com/asia/2020/04/11/not-even-singapore...

  All the while efficient contact-tracing teams—including 
  members of the police and the army—identified and isolated 
  thousands of people possibly infected with the virus. 
  Members of the armed forces have been making up to 2,000 
  calls a day to hunt for potential carriers. Those told to 
  stay at home for 14 days have been monitored assiduously 
  to ensure compliance. (Unco-operative types face 
  prosecution or the loss of their residency rights, if they 
  are not citizens.)

  Yet in spite of everything, the virus continues to spread.
  Singapore’s approach continues to evolve. Take face masks. 
  Initially Singaporeans were advised that they did not need 
  to wear them unless unwell. Then on April 3rd, in his 
  third televised address on covid-19, Lee Hsien Loong, the 
  prime minister, said that the government would no longer 
  discourage their use and would, in fact, distribute 
  reusable ones to every household. Singapore’s testing 
  regime may alter too. Currently people’s travel history 
  and symptoms are among the factors considered before they 
  are tested for the coronavirus. But health officials say 
  the approach is reviewed regularly and that wider testing 
  might be adopted in future. 

The difficulty with SARS-CoV-2 is once exposed, there's a window of several days of asymptomatic contagious transmissions. This is a biological health problem, and an app won't stop the spread. Viral fomites are like dust particles, airborne and remaining infectious on surfaces for upwards of 1-9 days depending on environmental factors [2].

Epidemiologists have predicted a global pandemic would happen sooner or later [3], and without a safe treatment the most effective action is to use Non-Pharmaceutical Interventions in any region where the virus might spread. Obviously due to modern air travel, it's now nearly everywhere.

[1] https://www.economist.com/asia/2020/04/11/not-even-singapore...

[2] https://news.llu.edu/health-wellness/covid-19-virus-remains-...

[3] https://wwwnc.cdc.gov/eid/article/26/5/19-0995_article

https://stacks.cdc.gov/view/cdc/11425

https://stacks.cdc.gov/view/cdc/45220

https://cme-mec.ca/wp-content/uploads/2020/03/CME_Pandemic_G...

https://www.ecdc.europa.eu/sites/portal/files/documents/Guid...