Vietnam’s Ministry of Health confirmed 11 community-based COVID-19 cases on Monday evening, all linked to a hospital.
The cases include four health workers and seven patients at Da Nang Hospital, located in the eponymous city in the central part of the country, the health ministry said.
Their ages range from 24 to 70.
The new patients were tested on Sunday and the results came back positive on Monday.
They are isolated for treatment at Da Nang Hospital, which has been locked down alongside Da Nang C Hospital as well as Da Nang Orthopedics and Rehabilitation Hospital, for infected patients have visited the institutions.
Vietnam has reported 431 coronavirus patients, including 276 imported cases quarantined on arrival, according to the health ministry.
A total of 365 cases have recovered from the disease while zero deaths have been documented in the country.
The Southeast Asian nation has logged 15 locally-transmitted infections since Saturday, ending 99 days of no community transmission.
Fourteen are treated in Da Nang while one is in medical care in Quang Ngai Province, about 160km south of Da Nang.
Da Nang has enforced strict social distancing in certain parts to prevent the virus from spreading.
The city has banned crowded events and shuttered non-essential services, while it will require the residents in these areas not to leave the home unless under necessary circumstances and to wear face masks whenever outdoors for at least two weeks starting Tuesday.
Acting Minister of Health Nguyen Thanh Long said at a meeting in Hanoi on Monday that the 15 patients have contracted a new virus strain detected for the first time in Vietnam.
The strain is more contagious than the five types having been identified in the country before, Long added.
But it is not clear if the new pathogen is more virulent, he noted.
Vietnam has barred entry to foreigners since March but foreign experts and skilled workers are still allowed to enter the country, subject to government approval and compulsory quarantine on arrival.
The government has sent planes to other nations to repatriate Vietnamese citizens, also quarantined upon landing.
Officials in Da Nang and its neighbor Quang Nam Province have detected dozens of Chinese illegally entering Vietnam and staying in local lodging facilities.
The Da Nang Department of Police confirmed on Sunday they had coordinated with their counterparts in Quang Nam to arrest Gao Liang Gu, 42, at a hotel in Ngu Hanh Son District, Da Nang.
Gao is a member of a ring that organizes the illegal entry of Chinese nationals into Da Nang and Quang Nam, police said.
Vietnam is quarantining 11,954 people who had close contact with infected patients or entered the country from virus-hit regions, according to the National Steering Committee for COVID-19 Prevention and Control.
The nation has 118 COVID-19 labs, including 66 authorized to confirm positive cases, the health ministry said.