Australia’s New South Wales (NSW) state reported zero coronavirus infections for a 4th straight day on Monday, but fears about new cases persisted as the missing link in a case that has reinstated curbs continued to elude officials.

Australia’s most populous state on Sunday extended social distancing restrictions in Sydney by a one-week period after authorities could not find a transmission path between an infected overseas traveler and a resident in his 50s who tested positive last week.

NSW state Chief Health Officer Kerry Chant told reporters in Sydney that “there has been a lot of work to identify how the transmission event occurred. Unfortunately, we haven’t found that missing link.”

The man may have contracted the coronavirus through brief contact with a currently unidentified person who was infectious, Chant said.

“So, because of that, we are still concerned that there may be chains of transmission in the community that are yet unrecognized,” she added.

Over 5.3 million people residing in and around Sydney, Australia’s biggest metropolitan area, have been ordered to wear face masks on public transport and at indoor venues, while house gatherings are limited to twenty guests until May 17.

The New South Wales  opened on Monday a mass vaccination hub in Sydney aiming to administer up to 30,000 COVID-19 vaccines per week to ramp up a national inoculation program that is behind schedule.

With just over 29,900 coronavirus infections and 910 fatalities, Australia has fared much better than several other developed countries but its countrywide immunization drive has hit roadblocks.