Downloads of Japan's coronavirus contact-tracing app have slowed since its debut last month, a blow to the government's aim to use smartphones to head off a second wave of infection.

Just 27 positive cases have been registered through the app, named COCOA for Contact-Confirming Application, even as Tokyo saw new daily cases surge to almost 300 in recent days.

Downloads stood at 7.69 million as of Monday, said health ministry official Yasuyuki Sahara. In the first week after its June 19 start, downloads had reached 4.4 million.

An early bug in COCOA prevented users from registering positive coronavirus test results. The health ministry said the bug was fixed with a July 13 update.

The app was designed by Microsoft Corp and is available for Apple Inc.'s iPhone as well as devices using Google's Android software.

It uses Bluetooth signals to detect contact with nearby users lasting 15 minutes or more. If a user later tests positive for the virus their contacts can be traced and notified through the program.

Numerous countries have rolled out contact-tracing apps, including Australia, Malaysia, the U.K., India, Germany and Italy. Singapore was among the first with its TraceTogether app launched in March, but privacy concerns hampered its uptake, prompting a switch to wearable devices.