New top story from Time: Vietnam Went Nearly 100 Days Without a Local Coronavirus Case. A New Outbreak Has Shattered the Calm

After almost 100 days without a new local patient, Vietnam had all but claimed victory over the novel coronavirus.

Now the masks are back on as the government works to contain a sudden flare-up in community infections after officials reported 15 new cases in three days from the central coast region.

The startling news, which began Friday with one suspected case, underscored the unpredictable nature of the virus. Vietnam is joining a number of other regions that had appeared to control outbreaks, including mainland China, Germany, South Korea and Hong Kong, in combating new clusters of unknown origin in an effort to head off raging outbreaks.

Vietnam is throwing every weapon in its arsenal at its effort to contain the new spread in Danang. Authorities announced four cases over the weekend and 11 more Monday night, which were tied to a Danang hospital where other virus patients have been treated. Four of the 11 are medical staff.

Vietnam’s first case since April 16 was perhaps the most unnerving: a 57-year-old grandfather who had been in Danang for about a month and hadn’t traveled to other provinces. That has triggered concerns that Vietnam, which has reported a relatively low 431 infections overall, may be vulnerable to imported cases from people crossing the border illegally into the country.

Prime Minister Nguyen Xuan Phuc has ordered tighter border and immigration controls. Police have made a number of arrests of Vietnamese and Chinese charged with shepherding people into the country illegally.

Still, Vietnamese health officials, like their counterparts elsewhere, are puzzling over the source of new infections. Hong Kong, for instance, had enjoyed three months of few local clusters before a surprise resurgence in recent weeks that has turned into its worst wave so far. Infections of unknown origins remain high at about 40% of new cases, reflecting that hidden chains of transmission continue.

So Vietnamese leaders are doing what has succeeded in the past: deploying swift and blunt lockdown-like actions.

Danang’s authorities shut down two hospitals where two victims visited and ordered patients, medical staff, caregivers and family members — about 7,000 people in total — to be quarantined for 14 days, local media reported. A third hospital was also locked down, VnExpress reported.

The city issued a stay-at-home order for 15 days for six of eight districts starting Tuesday, the local government said on its website. Residents are prohibited from going out except for essential reasons like buying food and medicine, for health emergencies or working at factories. Residents must keep a two-meter separation from each other.

Furthermore, operations of private vehicles are being restricted. And the government announced Tuesday that all domestic passenger flights to and from Danang will be suspended for 15 days.

Meanwhile, other parts of the country are taking precautions. In Hanoi, some 790 kilometers to the north of Danang, officials reissued anti-virus mandates — including wearing masks in public areas.

(Updates with wave in Hong Kong, elsewhere; Danang flight suspension)

–With assistance from Mai Ngoc Chau.