Two residential blocks in Yau Tong and Aberdeen were put under overnight lockdown on Wednesday after the discovery of preliminary-positive cases there.
Authorities said residents at Maya Tower 1 in Yau Tong had to be tested after a 19-year-old secondary student living there tested preliminary positive for a mutant Covid strain.
They have also locked down Pik Long House in Shek Pai Wan Estate in Aberdeen and ordered its residents to get tested.
A 20-year-old University of Hong Kong student who lives there has tested preliminary positive.
Health authorities expect to complete their operations by 7am on Thursday.