But two more treated patients relapsed on Wednesday evening, bringing the total number of relapses to 11. Of the 270 confirmed cases, 51 are now active while the remaining 219 people have been discharged from hospitals. 14 have tested negative at least once.

Deputy Prime Minister Vu Duc Dam on Wednesday instructed the Ministries of Science and Technology and Health and other related agencies to research to develop a vaccine against the disease.

Medical experts have yet to identify the causes of relapse. The health ministry has said a weak immune system could be the cause, enabling the virus to remain in the body.

It said on Tuesday that tests of samples taken from five relapsed patients showed the virus is "inactive" in their bodies, which theoretically means the chances of them spreading the infection to others are "very low."

Other countries in the world have also reported relapses. In South Korea, at least 222 people have tested positive for the virus after recovering. China and Brunei have reported relapses too.

Some 42,000 people are in quarantine in Vietnam, 323 in hospitals, 6,600 at quarantine facilities and the rest at home.

The health ministry said on Wednesday it had noticed Realtime PCR machines for testing Covid-19 selling at abnormal prices in some localities and asked the Ministry of Public Security to investigate.

The pandemic has spread to 210 countries and territories, claiming more than 227,000 lives.

The time between exposure to the new coronavirus and manifestation of symptoms is usually five to six days but can range from one to 14 days, according to the World Health Organization.