The 40-year-old man had been declared Covid-19 free for the first time on April 16 and was kept at the Quang Ninh Field Hospital No.2 for further monitoring. Five days later, his sample tested positive again, and his quarantine and treatment continued.

Repeat test results from April 28 to May 4 have all been negative. Doctors said he is in stable health, has no fever, cough or trouble breathing.

On Thursday, another relapse case, a 44-year-old woman, was also declared Covid-19 free for the second time. She was an employee of the Truong Sinh Company, which provided food and logistic services for several Hanoi hospitals, including the Bach Mai Hospital, which became a Covid-19 hotspot.

She was confirmed free of the virus for the first time on April 16, and found to have relapsed two days later.

In other news, "Patient 91", Vietnam’s most seriously ill Covid-19 patient, was transferred from the HCMC Hospital of Tropical Diseases to Cho Ray Hospital Friday evening. His condition remains critical and he is still on life support. In the past 19 days, he has tested negative for the virus five times.

"Patient 91" is a British man who works as a pilot for national flag carrier Vietnam Airlines. He was admitted to the hospital on March 18.

Doctors said he suffers from a blood clotting disorder and cytokine storm syndrome, an intense immune response where the immune system releases a lot of cytokines through the bloodstream, which works against the body.

The HCMC Hospital for Tropical Diseases announced Friday that 30 percent of his lungs could now function after 65 days of treatment, compared to just 10 percent a week ago.

However, hospital director Nguyen Vinh Chau said a lung transplant is still on agenda, as decided by the Health Ministry last week. The ministry also considers transporting him to the U.K. when he's well enough.

Vietnam has been through 36 days without community transmission. So far, the country has recorded 324 Covid-19 cases of whom 267 have recovered. Among the 57 active cases, six have tested negative once and three others twice.

At the moment, 14,000 people are under quarantine – 266 at hospitals, 8,000 at quarantine centers and the rest in their homes.

The Covid-19 pandemic has affected 213 countries and territories so far, with the reported death toll crossing 335,000.