The substitution was only discovered the next morning, Hoang Nam, Vice Chairman of Quang Tri People's Committee said Monday.

"He flew on the same flight with ‘patient 30’ from Hanoi to Hue, and he was supposed to be quarantined but sent his staff as replacement," Nam said.

The unnamed man is the business head of a company that invests in a wind power project in the central province of Quang Tri.

After learning of the substitution, authorities requested the man to present himself for quarantine. He has since been quarantined at the Tuberculosis and Lung Specialist Hospital in Dong Ha Town in the province.

At 21:30 Friday, four men checked into a hotel in Huong Hoa District, 65 kilometers west of Dong Ha Town. They were identified as co-passengers of Vietnam’s 30th Covid-19 infection on flight VN1547 from Hanoi to Thua-Thien Hue.

The men were placed in quarantine Sunday. Authorities only discovered there was a replacement the next morning.

Huong Hoa District authorities are tracking down people the men had come in contact with so as to place them in quarantine, too. They will also disinfect places visited by the suspects.

The 30th Covid-19 infection was a 66-year-old British woman who has been quarantined in the central town of Hue since Saturday. She was among 10 foreigners who have tested positive for the virus – all of them co-passengers on the same flight as Hanoian Nguyen Hong Nhung, who tested Covid-19 positive Friday.

Nhung, 26, who had flown to Hanoi from London on Vietnam Airlines flight VN0054, became the capital city's first Covid-19 patient and the nation’s 17th.

There were 201 passengers on the flight VN0054, including 21 in the business class, Vietnam Airlines General Director Duong Tri Thanh said Saturday. Of the 21, 18 were foreigners.

On Sunday, it was confirmed that a 61-year-old Hanoian on the same flight as Nguyen Hong Nhung had contracted the novel coronavirus. The man, the nation’s 21st Covid-19 infection, lives in the same neighborhood as Nhung in Ba Dinh District.

City authorities have been working with Hanoi’s Noi Bai Airport to collect information about 217 passengers and crew members on the flight.

As of Monday, the national total of confirmed Covid-19 infection had risen to 31.

The Covid-19 outbreak has thus far spread to 104 countries and territories around the world, with the death toll climbing to 3,806.