Hong Kong authorities on Monday said that a patient had received blood from a donor who later tested positive for the coronavirus.
Officials were scrambling to trace what happened to the blood after it emerged a man who donated on July 5 was later found to be carrying the virus.
"The blood transfusion service has been tracing the blood he donated and for now found that a patient in the Queen Elizabeth Hospital received a platelet transfusion," Lau Ka-hin, from the city's Hospital Authority told reporters.
It was not clear whether the 35-year-old male recipient had contracted the virus.
"The patient who received the transfusion has been transferred to an isolation ward and is being tested as the hospital continues to follow up on the case," Lau said, adding the patient had an ongoing chronic condition.
Hong Kong was one of the first places to be struck by the coronavirus when it emerged from central China.
The city has had impressive success in tackling the disease with just over 1,400 infections and seven deaths. In the last two months local transmissions had all but ended.
However in the last two weeks a cluster of local infections has emerged, prompting authorities to reintroduce some social distancing measures.
On Monday a major book fair due to open later this week was postponed because of the new spike.