HONG KONG - Hong Kong's Center for Health Protection reported four new cases of COVID-19 on Friday, taking the total tally to 11,774.

Among the newly reported cases, two were imported and one was a local case with unknown source, while the remaining one was epidemiologically linked with another local case confirmed earlier, the CHP said.

The case with unknown source, which involved a 39-year-old female foreign domestic helper, was confirmed on Thursday the first untraceable local infections of the highly transmittable N501Y mutant strain of novel coronavirus.

To stop the potential risk of spread of N501Y mutant strain, the CHP on Thursday night arranged compulsory quarantine for asymptomatic residents of the building in Tung Chung of Lantau Island where the above case resided.

Sophia Chan, secretary for food and health of the Hong Kong Special Administrative Region government, said at a press briefing on Friday that as a precaution, the HKSAR government decided to require all foreign domestic helpers in Hong Kong to undergo compulsory testing before May 9. Those who have received two doses of COVID-19 vaccine for more than 14 days will be exempted.

Hong Kong launched a COVID-19 vaccination drive on Feb 26. About 897,100 people have received the first dose of vaccine, and about 480,500 have received the second so far.

Chan said that the two vaccines available in Hong Kong are effective against the mutant strain and urged the public to be vaccinated as soon as possible.