The 48 active patients include eight relapse cases. Three relapses were recorded Monday - two in Ho Chi Minh City and one in the northern province of Phu Tho. All the relapsed patients are being monitored at different hospitals in the country.

Of the 48, eight have tested negative once and six twice.

The eight relapses exclude one patient, a Brit who has left Vietnam and was confirmed negative in the U.K.

Three of the eight relapsed patients tested negative again Monday, including two in the northern province of Quang Ninh and one in the central province of Binh Thuan. All three were in stable health, doctors said.

While there is no definitive answer yet on why the relapses are occurring, Nguyen Trong Dien, Director of Quang Ninh's Health Department said: "It's possible that the patients' immune system has weakend, causing the virus to flare up again because there is still no specific medicine to treat the novel coronavirus."

Doctor Nguyen Thanh Binh, deputy head of the general planning department at the National Hospital for Tropical Diseases in Hanoi, said doctors have only confirmed that the patients have tested positive again but there was not enough evidence to confirm they have been reinfected.

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

Vietnam have recorded 270 coronavirus patients so far, of whom 222 have been discharged from hospitals.

By last Friday morning, the country had gone eight straight days without a new infection, but the evening saw two new cases, both students returning from Japan. They were quarantined on arrival and are now under treatment in Hanoi.

There have been no cases of community transmission since April 16.

The country has 52,000 people in quarantine at present - 323 in hospitals, 11,000 in quarantine facilities and the rest at home.

The Covid-19 pandemic has spread to 210 countries and territories, with almost 207,391 deaths reported so far.