Port Centre was locked down overnight after one resident tested preliminary positive for a mutant strain of Covid-19.
The Centre for Health Protection said the 41-year-old woman works as a part-time cleaner at the Bridal Tea House in Yau Ma Tei – a quarantine hotel.
She has no recent travel history. Four imported Covid-19 cases were also reported on Thursday.
The patients, aged between 19 and 54, arrived from the UK, Russia and Indonesia.