Hoang Duc Hanh, deputy director of the Hanoi health department, said the city hosts a high number of people from northern Hai Duong and Quang Ninh provinces, where a swath of community transmissions has been confirmed. Hai Duong and its neighbor Quang Ninh are only around 60 kilometers from Hanoi.

The Hai Duong hotspot is particularly troublesome due to the presence of a coronavirus variant originating from the U.K., believed to be up to 70 percent more contagious than ordinary strains, he added.

"As such, Hanoi faces a high risk of detecting new community transmissions. Without good management, the capital could become a Covid-19 hotspot," he said, adding authorities need to roll out preventative measures at the highest levels.

The Hanoi Center for Disease Control (CDC) said the coronavirus might have been lurking within Hai Duong for at least 10 days, meaning Jan. 14 should be a good marker to determine appropriate quarantine measures in the city.

"Bac Giang, Hung Yen, Hai Phong – all localities close to the capital have traced people who are associated or had come in contact with the confirmed coronavirus cases in Hai Duong and Quang Ninh," said Truong Quang Viet, deputy director of the Hanoi CDC.

Authorities have now requested anyone entering Hanoi who have been to existing Covid-19 hotspots since Jan. 14 to self-isolate for 14 days. Those who have been to Hai Duong, Quang Ninh have to do so as well. Other measures like mask wearing and keeping a safe distance must also be applied.

Hanh predicts future cases in Hanoi might sprout up from hospitals, where the ill would typically turn up for help. As such, hospitals must also step up coronavirus prevention measures, starting with collecting samples from any patient from Hai Duong and Quang Ninh admitted to hospital since Jan. 14 for Covid-19 testing, those with severe or underlying conditions like hypertension or kidney failure, and those with respiratory symptoms, he added.

As of Friday morning, Hanoi has locked down an apartment building of the Times City urban complex and isolated an alley on Cau Giay District's Nguyen Khanh Toan Street, home to two coronavirus patients.

Thousands of studentshave also stayed home starting Friday. They included Phan Dinh Phung High School in Ba Dinh District, with over 2,000 students, where one student and two parents had been in close contact with a coronavirus case. The Vinschool kindergarten, secondary and high schools located within Times City have also let students stay home. Several universities in the capital have since followed suit.

Vietnam's Health Ministry has confirmed 93 Covid-19 community transmissions since Thursday, recorded in Hai Duong, Quang Ninh, Hai Phong, Bac Ninh and Hanoi.

Hai Duong is now the biggest hotspot with 77 cases, followed by Quang Ninh with 13 cases, and one each in Hanoi, Bac Ninh and Hai Phong. Hai Duong’s Chi Linh Town, with around 220,000 residents, has been put under lockdown for 21 days starting noon Thursday to curb the spread of the virus.

So far, 1,651 cases have been recorded nationally, with 35 deaths.