They noted the older Chinese man had become ill on January 17, four days after he first arrived in Hanoi. On January 22, he was admitted to hospital in Ho Chi Minh City with fever and fatigue. His wife who travelled with him showed no symptoms.

The man tested positive for the coronavirus although he told doctors he had not been exposed to any “wet market”. Experts initially believed a seafood market in Wuhan was the epicentre of the contagion but questions have since emerged about the accuracy of this assessment after studies indicated not all infections were linked to the market.

At Ho Chi Minh City’s Cho Ray Hospital, the man was isolated and treated with antiviral agents, and his condition had improved by last weekend.

However, the second case involving the man’s son raised concerns about person-to-person transmission, the doctors wrote.

The 27-year-old man had been living in Long An, 40km south of Ho Chi Minh City, since October last year and had not had contact with anyone else returning from the Wuhan region. He met his parents in the coastal resort town of Nha Trang on January 17 and they shared a hotel room for three days.

He began to feel ill on January 20 and two days later on January 22 arrived with his father at Cho Ray Hospital in Ho Chi Minh City with a fever. He was also isolated and subsequently diagnosed with 2019-nCoV.

“This suggests that the incubation period for 2019-nCoV may have been three days or less in this case,” the doctors wrote in the NEJM.

“His father was thought to be source of the infection. However sequencing of strains from the two patients to ascertain transmission has not been performed,” the doctors added.

The family had visited four Vietnamese cities, taking plans, trains and taxis.

“A total of 28 close contacts have been identified, and symptoms of an upper respiratory infection have not developed in any of them,” the doctors wrote, but said “this family cluster of 2019-nCoV infection that occurred outside China arouses concern regarding human-to-human transmission”.

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Vietnam’s health ministry’s online portal on Tuesday said no new confirmed cases had been reported but 64 people had been identified as suspected cases. Among them, 25 had tested negative for the virus while the rest had been quarantined.

Cho Ray Hospital on Monday said the younger man, whose name is Li Zhichao, had recovered while his father Li Ding, was still being quarantined.

Although none of the casualties from the virus has been recorded outside China, concerns are growing around human-to-human transmission.

The virus can cause pneumonia and, like other respiratory infections, it spreads between people in droplets from coughs and sneezes. It is too early to know what its death rate will be, since there are likely many milder cases going undetected.

Wuhan virus: Asia-Pacific countries weigh restrictions against Chinese travellers

Vietnam has increased screening measures of all travellers entering the country, with Ho Chi Minh City-based travel guide Le Van Anh Khoa saying he had never seen an epidemic like this in his five years working in the tourism industry.

The 26-year-old said his company had suspended outbound tours to mainland China, while local media reported Vietnam’s tourist hubs of Da Nang, Khanh Hoa and Lao Cai had asked travel firms to stop taking in Chinese tourists and to cancel outbound tours to countries where infections had been reported.

Last year, Vietnam welcomed 5.8 million Chinese tourists, a 17 per cent increase compared to 2018.

“Since the end of December, many tourists are also concerned for their health and cancelled the trips with us,” Khoa said. “I want safety for my customers, but even I’m worried for myself too.”