Effectiveness of COCOA, a COVID-19 contact notification application, in Japan

Background COCOA, a contact reporting application in Japan, was launched at the end of June 2020. Object We assessed effectiveness of COCOA. Method After developing a simple susceptible–infected–recovery model with COCOA and voluntary restrictions against going out (VRG), we assumed that COCOA can reduce infectiousness by 10–50% points through self-quarantine at home after receiving notification from COCOA. Results COCOA alone is insufficient to halt an outbreak. Even if the entire population were to use COCOA, the reproduction number would be 1.31. However, if VRG were 15%, about half of the maximum VRG effectiveness under the emergency state declaration, then 10% COCOA use by a population can reduce the reproduction number to less than one. Conclusion Significant effects of COCOA for reducing the reproduction number were found. However, without VRG, COCOA alone is insufficient to control an outbreak.