An announcement of the two having recovered after relapsing is expected Tuesday. They will be the first among the 14 relapse cases to have recovered.

A spokesperson for the National Hospital of Tropical Diseases in Hanoi said Monday that relapsed cases "Patient 74" and "Patient 137" have tested negative four times in a row. The sample results also show the virus have not developed and are inactive.

Doctor Nguyen Thanh Binh, Deputy Head of the General Planning Department, formerly the secretary of the Covid-19 treatment team at the hospital, said the real-time RT-PCR tests, a technique that combines reverse transcription of RNA into DNA and amplification of specific DNA targets using a polymerase chain reaction (PCR), can be very effective.

Sometimes, even if only "a strand of the virus’s RNA" remains in their sample, the test result will be positive, but this does not provide certainty that the patient is indeed positive for the coronavirus," Binh said.

"Results of the virus being cultured are the most accurate evidence, confirming that the relapse cases no longer have pathogenic viruses in their bodies and are unable to transmit the disease among the community," he said.

Doctor Pham Quang Thai, Deputy Head of Infection Control Department at the National Institute of Hygiene and Epidemiology, said: "A positive result on a PCR test means that you find genetic material of a virus, which does not mean the virus is alive and still causing the disease."

In addition to PCR tests, experts at the institute have also conducted additional tests in the relapsed cases, including checking them for antibodies and cultivating the virus to see if it replicates or not. So far, there is no evidence that shows relapsed cases carry the possibility of being infectious, the experts have concluded.

"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.

"Patient 137," a 34-year-old man in the north central province of Nghe An, who was released from Hanoi April 7, was also quarantined for another 14 days. He was confirmed positive a day after he returned to his hometown.

"However, after recovery, patients should still be isolated in hospitals for doctors to monitor and do their research to better understand the mechanism of the pathogen of the virus, which is very new to medicine," doctor Thai said.

Three hypotheses

Doctor Le Quoc Hung, Head of the Department of Tropical Diseases at the Cho Ray Hospital in HCMC, said there have been three main hypotheses about Covid-19 relapses. Patients either relapsed after getting re-infected, the testing was wrong, or the virus was inactive.

The majority of relapsed cases in Vietnam do not show clinical symptoms and are detected with the virus after being tested after being discharged as part of the medical protocol. Only one of the 14 relapse cases seen so far shows mild symptoms, but this could be because the patient was infected with other viruses. People who have come into close contact with relapsed cases have all tested negative and there has not been a community transmission, Hung noted.

"The fact that the relapses show no clinical symptoms and have not spread to those in close contact shows a very low possibility of them being re-infected," Hung said.

However, experts have not been able give a concrete conclusion because it takes at least three weeks to see the results of experiments where the coronavirus is cultured using samples of the relapse cases.

Several other countries have also reported relapses. In South Korea, more than 260 relapse cases have not infected others. Experts at the Central Clinical Committee for Emerging Disease Control believe that RNA can remain in patients after the virus has been inactive, so remnants of the virus can still be found.

In China, more than 30 people in Hubei Province have also relapsed after being discharged from the hospitals. Zhang Boli, President of Tianjin University of Traditional Chinese Medicine, said the genetic sequence remaining in some patients showed that the virus was inactive.

This means it is possible that there is only genetic material of the virus in the body, Zhang said.

Vietnam’s Health Ministry confirmed no fresh Covid-19 cases Tuesday morning, marking the country's 19th straight day without community transmission.

The country has recorded 271 coronavirus cases to date, of whom 50 are active infections. Of the 50, 12 are relapses.