Chuang Shuk-kwan, head of the Centre for Health Protection's Communicable Disease Branch, said around 30 to 40 percent of the local cases recently have their source of infection remain unknown, it is essential for the authorities to step up their contact tracing and stop any possible transmissions.

Apart from close contacts of the patients needing quarantine, if those close contacts also developed symptoms, their household members will also have to be put under quarantine for three days.