Dr Nguyen Tri Dung, director of the HCMC Center for Disease Control, said earlier the city only tested people on the first and last days in accordance with national Covid-19 safety protocols.

"The decision to take samples four times for testing stems from HCMC’s evaluation of people entering the city during this period having a higher risk of nCoV infection than before," Dung said.

HCMC and Vietnam's latest case, "Patient 271", diagnosed on Sunday, is a British oil expert who arrived in HCMC from the U.K. in a private jet to work on a Petrovietnam project.

He was quarantined on arrival on April 28 and tested negative. But a test five days later came out positive.

He has been moved to the city's Cu Chi Field Hospital for treatment.

He arrived along with 12 other experts, all of whom have tested negative so far.

The city has recorded a total of 55 cases. Six patients relapsed recently after being discharged, taking the number of active cases to eight. The rest recovered and were discharged from hospitals.

Authorities will monitor recovered patients for 30 days after discharge instead of the earlier 14 days, and deploy measures to prevent relapses.

They also monitor international flight crews arriving in the city, and have taken samples from 280 people so far.

Vietnam began day 18 without community transmission on Monday.

The country has 27,409 people in quarantine, including those who arrived from abroad and those who had come into contact with patients. Of them, 238 are in hospitals meant for Covid-19 treatment, 5,871 are in public quarantines and 21,300 are isolated at home.

The pandemic has spread to 212 countries and territories, and claimed more than 248,000 lives.