A Chinese man became ill from a novel coronavirus (2019-nCoV) 4 days after arriving in Vietnam to visit his 27-year-old son. Three days later the healthy young man was also stricken, according to a report published online today in the New England Journal of Medicine.

"This family cluster of 2019-nCoV infection that occurred outside China arouses concern regarding human-to-human transmission," the authors write.

The father, age 65 years and with multiple comorbidities including hypertension, type 2 diabetes, coronary heart disease with stent placement, and lung cancer, flew to Hanoi with his wife on January 13; they traveled from the Wuchang district in Wuhan, where outbreaks of 2019-nCoV have been occurring.

On January 17, the older man and his wife met their adult son in Ho Chi Minh City and shared a hotel room with him for 3 days. The father developed a fever that same day and the son developed a dry cough, fever, diarrhea, and vomiting on January 20. Both men went to a hospital emergency department on January 22.

The authors say the timing of the son's symptoms suggests the incubation period may have been 3 days or fewer.

Upon admission to the hospital, the father reported that he had not visited a "wet market" where live and dead animals are sold while he was in Wuhan. Throat swabs were positive for 2019-nCoV on real-time reverse-transcription–polymerase-chain-reaction assays.

The man was placed in isolation and "treated empirically with antiviral agents, broad-spectrum antibiotics, and supportive therapies," write Lan T. Phan, PhD, from the Pasteur Institute Ho Chi Minh City in Vietnam, and coauthors.

On admission, chest radiographs revealed an infiltrate in the upper lobe of his left lung; he developed worsening dyspnea with hypoxemia on January 25 and required supplemental oxygen at 5 L/min by nasal cannula. Chest radiographs showed a progressive infiltrate and consolidation. His fever resolved on that day and he has progressively improved.

The man's son had a fever of 39°C (102.2°F) when the two men arrived at the hospital on January 22; hospital staff isolated the son, and chest radiographs and other laboratory tests were normal with the exception of an increased C-reactive protein level.

The son's throat swab was positive for 2019-nCoV and he is believed to have been exposed from his father; however, the strains have not been ascertained.

"This family had traveled to four cities across Vietnam using various forms of transportation, including planes, trains, and taxis," the authors write. A total of 28 close contacts were identified, none of whom have developed respiratory symptoms. The older man's wife has been healthy as well.