Only a third of infections could be found if authorities rely just on clinical symptoms, Nguyen Nhat Cam, director of the Hanoi CDC, said, warning even asymptomatic people could spread the disease.

"Monitoring and testing are vital in preventing Covid-19."

Hanoi's strategy from the very beginning has been to detect and monitor cases early, including through performing Covid-19 tests, so that outbreaks could be extinguished quickly, he said.

"Thanks to this strategy of actively collecting samples for testing, Hanoi has detected several sources of infection in the community such as in Ha Loi Village."

Ha Loi, with nearly 11,000 residents, was placed under a lockdown on April 7 after at least three people tested positive for the virus. As of Monday morning at least 10 people in the village have been confirmed positive.

The village has emerged as the second biggest hotspot in the capital after Bach Mai Hospital. The hospital, linked to at least 45 cases, went into a two-week lockdown, which was lifted on Sunday.

Hanoi leads the nation in the number of Covid-19 cases, 125, of whom 58 have recovered and been discharged. The capital has performed mass rapid testing at several locations since March 31.

Vietnam has had 262 confirmed cases, 117 of them active.

The pandemic has so far spread to 210 countries and territories and claimed more than 114,300 lives.