  How to avoid a second wave of COVID-19 after reopening the economy is a
pressing question. The extremely high basic reproductive number $R_0$ (5.7 to
6.4, shown in new studies) of SARS-CoV-2 further complicates the challenge.
Here we assess effects of Social distancing 2.0, i.e. proximity alert (to
maintain inter-personal distance) plus privacy-preserving contact tracing. To
solve the dual task, we developed an open source mobile app. The app uses a
Bluetooth-based, decentralized contact tracing platform over which the
anonymous user ID cannot be linked by the government or a third party.
Modelling results show that a 50\% adoption rate of Social distancing 2.0, with
privacy-preserving contact tracing, would suffice to decrease the $R_0$ to less
than 1 and prevent the resurgence of COVID-19 epidemic.
