He said with the new strain, each infected person may infect about five to six people compared to 1.8 to 2.2 people in the previous period.
Authorities have taken a series of "unprecedented measures" to fight the outbreak in Danang, including the mobilisation of several hundred military school students to help with contact tracing and collecting test samples, the government said in its statement.
Dr Kidong Park, the World Health Organization's representative in Vietnam, told Reuters the country had been preparing for the possibility of wider community transmission, after the country reported its first case in January.
"The government has always been determined to ensure that its people are protected from COVID-19 by keeping the country’s relatively low number of cases and controlling the transmission within the community," Park said in an emailed statement.
Authorities said on Saturday they plan to test Danang's entire population of 1.1 million people for COVID-19.
The Health Ministry said that up to 800,000 visitors to Danang have left for other parts of the country since Jul 1, adding that more than 41,000 people have visited three hospitals in the city since.
Stay safe