One of the relapse cases, "Patient 74," is a 23-year old Vietnamese man from the northern province of Phu Tho. He was confirmed positive for Covid-19 March 18 and discharged from the Bac Ninh General Hospital April 10 after treatment. He returned home in Phu Tho and isolated himself at home for another 14 days. During this period, he showed no symptoms of fever, cough or breathing difficulty.

On April 25, he had his samples taken for testing the last time before completing the 14-day home quarantine. However, the result came back positive and he was transferred to the National Hospital of Tropical Diseases in Hanoi.

Three family members in his home have tested negative, but been placed under quarantine. His home and neighborhood have been disinfected.

The other patients are "Patient 207" and "Patient 224", both Brazilian nationals linked to HCMC's largest Covid-19 hotspot, the Buddha Bar & Grill in District 2's Thao Dien Ward, home to a major expat community.

"Patient 207" is a 49-year-old Brazilian man who was confirmed positive March 31 and released from Cu Chi field hospital in HCMC April 18, while "Patient 224" is a 39-year-old Brazilian man confirmed infected April 2 and discharged from the hospital April 20.

On Sunday, both were taken back to the Cu Chi hospital for treatment after their samples tested positive for the novel coronavirus.

"Patient 151," a 45-year-old Brazilian woman and wife of "Patient 207", has also been transferred to the hospital for medical supervision due to being in close contact with him. She had been discharged on April 18.

Earlier, the nation had recorded six Covid-19 relapses. Two of them are "Patient 36," a 64-year-old woman in the south central province of Binh Thuan who was discharged April 10 and confirmed positive Friday on the verge of finishing her post-discharge quarantine; and "Patient 137," a 34-year-old man in the north central province of Nghe An, who was released from Hanoi April 7, quarantined for another 14 days, but confirmed positive again Thursday, a day after he returned to his hometown.

The three other relapse cases are: "Patient 188," a 44-year-old woman in Hanoi who tested positive for the novel coronavirus on April 18, two days after her discharge; "Patient 52," a 24-year-old Vietnamese woman returning from London; and "Patient 149," a 40-year-old Vietnamese man returning from Germany, who were both discharged April 16 and tested positive again five days later under quarantine at a hospital in Quang Ninh Province in northern Vietnam.

"Patient 22," a British man whose samples tested positive as he left Vietnam, has tested negative again in his home country.

With the latest three relapses, the number of Covid-19 active patients in Vietnam rose to 48 of 270 infections recorded so far.

The National Institute of Hygiene and Epidemiology and the Pasteur Institute in Ho Chi Minh City are cultivating viruses from relapse patients to further study the cause of the phenomenon.

Covid-19 relapses are not uncommon with South Korea reporting more than 200. The phenomenon was also reported on many cured patients in China.

Some health officials are assuming the phenomenon could result from the difference in viral load at different times or the difference in procedures and test devices.

However, leading experts and doctors are of the view that Covid-19 is a new disease about which many things have to be studied further.

The Covid-19 pandemic has spread to 210 countries and territories, with more than 207,000 deaths reported so far.