| A medical worker collects samples for COVID-19 tests in Hanoi, April 18, 2020. (Photo: VnExpress/Ngoc Thanh)
The 21-year-old man had returned to Vietnam from Russia and tested positive for the virus on August 25, and was treated at the National Hospital for Tropical Diseases in Hanoi.
After being discharged in mid-September he isolated himself at home for two weeks.
On November 7 he went to the Transport Hospital with a fever and was given medicines for a viral infection.
One week later he had a 39-degree fever and felt fatigued. He returned to the hospital and had a novel coronavirus test done, testing positive.
He is again at the National Hospital for Tropical Diseases for treatment. Authorities have yet to find out how he contracted the virus a second time.
A preliminary investigation found 25 people had been in close contact with him, including his grandmother, father and brother in Cau Giay District.
Hanoi had gone three months without a local case.