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

Kurita, J.; Sugawara, T.; Ohkusa, Y.

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.