Vietnam added seven local COVID-19 infections to its tally on Tuesday, including a health worker.
Four of the cases are citizens of Da Nang, a touristy city on the central coast, while the others hail from neighboring Quang Nam Province, the Ministry of Health said.
One of the new patients is a health worker at Da Nang Hospital, to which at least 16 cases have been traced since the weekend.
The hospital is now locked down alongside Da Nang C Hospital and Da Nang Orthopedics and Rehabilitation Hospital, as infected patients have been there.
Vietnam has confirmed 22 community-based cases since Saturday, ending a streak of 99 days without any local transmission of the virus.
Eighteen of the patients, including three critical cases, are in Da Nang, three in Quang Nam, and one in Quang Ngai Province.
Relevant government agencies are conducting aggressive contact tracing to prevent the virus from spreading further.
Da Nang started enforcing citywide social distancing on Tuesday that will be in effect for at least 15 days.
Authorities have banned public gatherings of more than two people, closed non-essential services, required citizens to wear face masks whenever outdoors, and asked them not to leave their home unless for essential trips while maintaining a two-meter distance in social interactions.
Flights to and from the beach city began to be suspended for 15 days on Tuesday.
Health workers will test about 10,000 people in Da Nang for COVID-19, a source told Tuoi Tre (Youth) newspaper.
Vietnam has detected 438 coronavirus patients as of Tuesday evening, with 276 imported cases quarantined upon arrival, according to the health ministry.
The nation has confirmed 369 recoveries and zero fatalities.
More than 15,000 people who had close contact with patients or entered Vietnam from outbreak-hit regions are being quarantined in the Southeast Asian country at the time of writing.
Vietnamese borders are still closed to foreigners but foreign experts and skilled workers are allowed in, subject to government approval and compulsory quarantine on arrival.
The government has sent aircraft to other nations to repatriate Vietnamese citizens, also quarantined upon landing.
Officials in Da Nang, Quang Nam, as well as other provinces and cities have detected dozens of Chinese illegally entering Vietnam and staying in local lodging facilities this month.