SINGAPORE - The Changi Airport Terminal 3 cluster began in the arrival gates and baggage claim hall, where airport staff worked in close proximity to arriving passengers, investigations by Changi Airport Group (CAG) have shown.

About half of airport workers who tested positive for Covid-19 were found spread out in the arrival zone, suggesting that areas where staff and arriving passengers were in close contact were the sites of the "primary infection", said CAG chief executive Lee Seow Hiang on Monday (May 24).

Those working in these areas then mingled with others working in the transit areas, departure gates and the Basement 2 foodcourt - "where the risk profile is no different than the rest of Singapore" - leading to secondary clusters of infection that eventually spread the coronavirus to more than 100 people.

Mr Lee noted that the other zones were "relatively clean".

"For instance, in the central transit area, there are about 2,000 people working there. By now, we have swabbed close to 97 per cent of them and there are zero cases, except for a specific cluster where investigations subsequently show there was a mingling of staff with those in the arrival zone," he said at a press conference.

A total of 43 positive Covid-19 cases have been detected among airport staff to date.

Of these, about half - 10 at arrival gates and 11 at the baggage claim hall - worked in the arrival zone at T3.

The secondary cluster that Mr Lee referred to, in the part of the transit area immediately bordering the arrival zone, has six cases.

Further from this boundary, a lone case has been detected at the departure immigration checkpoint in the transit area.

A key site of secondary spread that has been raised by Health Minister Ong Ye Kung is the Basement 2 foodcourt, where 12 of the 21 T3 arrival zone workers visited. Since then, 10 non-transit airport staff who visited the foodcourt have tested positive.

It is the first time staff have been infected in public areas in the airport , Mr Lee said.

"The public areas include offices, retail offerings, including Jewel - 8,000 people work in this zone. Similarly, close to 97 per cent of people have been swabbed. Zero positive cases, except the foodcourt.

"This foodcourt is subsidised for airport workers and is de facto the canteen for (those working in the arrival zone) and also serves the public. It is now quite clear that although there was a lot of attention on this, the foodcourt is not the issue.

"It is a secondary source of infection."

There are also two cases of staff who tested positive at an arrival gate in Terminal 1, which also handles flights from high-risk countries.

It is not known how they got the virus, but contact tracing showed that the two had also visited the T3 foodcourt. They could have been infected by arriving passengers or at T3.

The remaining three airport workers whose test results came back positive are likely to have been infected due to close contact with other airport workers outside the workplace, in their capacity as housemates or family members.

Last week, the Civil Aviation Authority of Singapore and CAG had said that the initial transmission that led to the T3 cluster could have occurred through an airport worker who was assisting a family from South Asia, who had arrived in Singapore on April 29.

Asked by reporters how exactly this happened, Mr Lee said: "I think we have disclosed what we know from a CAG point of view, and I don't want to speculate on the other details that are ongoing in the investigations.

"We are taking a complete, comprehensive look, based on the World Health Organisation and our own health authorities' understanding of the different modes of transmission. That's more or less the most important at the moment, over and above the micro-aspects of individuals."

He did not pinpoint any particular country as the source of infection.

It was revealed on Sunday that the South Asian family did not arrive on a flight from India. Test results for the initial batch of airport workers indicate the presence of the B1617 variant, which was first detected there.