The number of “infected people increased at a record figure” with 45 new cases, said the Suc Khoe Doi Song newspaper, a health ministry mouthpiece.
The patients aged between 27 and 87 were all being treated at or were employed at Danang’s hospitals, or were their relatives, it added.
State media said a makeshift hospital was being set up at a Danang sports center in anticipation of the outbreak worsening.
Flights in and out of Danang and public transport within the city have already been suspended, and the majority of its 1.1 million inhabitants have been advised to remain at home.
At least 21,000 people are undergoing rapid-test swabs for the virus in the capital Hanoi after traveling to Danang. Similar tests were also conducted in business hub Ho Chi Minh City for returnees from Danang.
Bars have been closed in nearby Hue, while tourism is suspended in historic Hoi An.
Vietnam has won praise for its handling of the virus — reporting just 509 infections in total, including those from the new wave, and until Friday no deaths despite sharing a long border with China.
Earlier this month state media trumpeted the survival of a British pilot who spent more than two months on life support in Vietnam with the virus.
Stephen Cameron, 42, was the sickest patient medics have had to treat during the coronavirus outbreak and was given just a 10 percent chance of survival.