One of them, a 20-year-old Vietnamese student who was studying in Wuhan, returned to Hanoi about a week ago. She has had fever and a painful throat for the last four days. The other, a 55-year-old merchant at a market near the Vietnam-China border, has had a high fever for the last five days.

Both are being quarantined at Hanoi's National Hospital of Tropical Diseases. The National Institute of Hygiene and Epidemiology is carrying out tests to determine the cause of her illness.

A pneumonia outbreak, suspected to be caused by a new strain of the coronavirus family that caused SARS and MERS, first surfaced in Wuhan City, China last December.

By January, the disease has spread to other countries and territories: Thailand, South Korea, Japan, Taiwan, Macau, Hong Kong and the United States.

China’s National Health Commission has confirmed 571 cases and 17 deaths as of Wednesday. It has also said that the virus is capable of spreading from person to person.

The World Health Organization (WHO) has said that an animal appears most likely to be the primary source of the virus. China says the virus could have come from a market in Wuhan where wildlife was traded illegally.

The Vietnamese Ministry of Health has warned people to keep their distance from people with acute respiratory infections and wear a mask when talking to them. It has also installed equipment to monitor arriving passengers’ body temperature in airports in Hanoi, Saigon, Da Nang and Cam Ranh in central Vietnam.