HONG KONG — Only six people in Hong Kong have tested positive for the coronavirus out of a batch of 128,000 residents who had undergone the mass-testing program that began on Tuesday.

Four of the six were previous coronavirus patients who had been discharged last month, and still carried traces of the virus when they were tested.

As of Thursday, 850,000 people in the city of 7.5 million had registered to take part in the weeklong program that offers all residents a one-time, free coronavirus test as the city seeks to identify silent carriers of the virus.

The low number of positive cases found so far has drawn criticism that the government’s universal testing program was not cost effective amid privacy concerns and fears that DNA data could be sent to mainland China.

Hong Kong saw its third and worst surge of coronavirus infections in early July. At its peak, Hong Kong recorded more than 100 local cases a day, after going weeks without any in June. Cases have steadily dwindled following a raft of tough restrictions, including limiting dining-in hours and shuttering businesses such as bars and karaoke lounges.

Apart from the six people, Hong Kong reported eight other coronavirus infections on Thursday. In total, it has reported 4,839 confirmed cases with 93 deaths.