"Patient 328" had landed May 13 at the Van Don International Airport in the northern province of Quang Ninh on Vietnam Airlines flight VN0062.
The boy was quarantined at a military facility in the nearby Hai Duong Province and tests by the Center for Disease Control and Prevention in Hai Duong came back negative three times in a row.
On Thursday, his fourth test returned positive and the very next day, the National Institute of Hygiene and Epidemiology in Hanoi said his sample had tested positive again.
He has been admitted to the Hospital for Tropical Diseases in Hai Duong for treatment.
A 62-year-old woman who accompanied the boy has already tested positive for the virus and is under treatment, too.
With the little boy, 34 people on board the flight from Russia have been confirmed to have contracted the novel coronavirus, two of them male flight attendants.
Vietnam now has 49 active patients and 279 have recovered, including a man who was announced Covid-19 free Friday evening.
The latest recovery is a 39-year-old man from Tan Chau District in Tay Ninh Province on the Cambodian border. He returned to Vietnam illegally from Cambodia via a trail on May 2 and was persuaded by his aunt to report his return to the police and be quarantined the next day.
Saturday morning marked day 44 that Vietnam has gone without recording a new coronavirus infection caused by community transmission.
The country’s most seriously ill Covid-19 patient, a British man who works as a pilot for national flag carrier Vietnam Airlines, was transferred from the HCMC Hospital of Tropical Diseases to Cho Ray Hospital last week after he was announced Covid-19 free.
"Patient 91" remains in critical condition and on life support, but some progress has been seen of late. On Friday, it was announced that 40 percent of his lungs could function, compared to just 10 percent two weeks ago. Comatose for around two months, he can now turn his head upon doctors’ request.
The Covid-19 pandemic has affected 213 countries and territories, with more than 366,300 deaths reported.