Vietnam was back on high alert for the coronavirus on Saturday after medical officials in the central city of Danang detected the country’s first locally transmitted case for three months.

Vietnam has imposed strict quarantine measures and carried out an aggressive and widespread testing programme during the pandemic, keeping its total tally of reported infections to just 417, with no deaths.

It had reported no locally transmitted infections for 100 days until Friday, when the Health Ministry said a 57-year-old man from Danang, a tourist hot spot, had tested positive.

State media reported late Saturday that the man was in critical condition, and specialised doctors had flown from Ho Chi Minh City to Danang to treat him.

Authorities said 50 people the patient came into contact with have been isolated. The ministry said 103 people connected to the patient were tested for the virus but all returned negative results.

It said more than 11,800 people are being quarantined throughout the country, including 147 at hospitals.

The government did not say how the man contracted the virus but said he had not left Danang for nearly a month. He was initially diagnosed with pneumonia. A fresh test on Saturday had confirmed the coronavirus infection, authorities said.

The case comes at a time when Vietnam was about to resume international commercial flights and as domestic tourism is surging.

Late on Friday, authorities in the capital, Hanoi, reinstated a recommendation to wear masks in public places.

Vietnam’s ban on international commercial flights is still in place, but foreign experts and skilled workers have been able to enter provided they undergo mandatory quarantine.

Of the nearly 150 cases reported over the past three months, all were found in people who had been quarantined on arrival.