Vietnam recorded its first locally transmitted COVID-19 case in 99 days on Saturday in the beach city of Da Nang, on the country’s eastern coast. By Monday, it had recorded three more local cases there, bringing Vietnam’s total to 420. Da Nang authorities have now reimposed social distancing measures and are preparing to evacuate 80,000 local tourists.

Vietnam had been a model country for its handling of the coronavirus. It closed borders, imposed lockdowns, set up quarantine facilities, and carried out rigorous contact tracing and testing early on in the outbreak. Its population of 96 million, the world’s 15th-largest, has recorded zero coronavirus deaths.

But its nascent outbreak again underscores the fragility of coronavirus success stories and the unpredictable nature of the COVID-19 pandemic.

Da Nang outbreak

Vietnam had not recorded a local case—an infection transmitted within the community, rather than imported from an inbound traveler—since April 16, and life was largely back to normal when a 57-year-old man in Da Nang tested positive for the coronavirus on Saturday. He hadn’t left the city or been in close contact with strangers, and authorities don’t yet know how he contracted the virus.

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Officials have since recorded three more local cases in Da Nang, a popular domestic tourism spot, and the government said on Monday that it is closing the city’s borders to tourists from other parts of Vietnam and evacuating the city’s 80,000 visitors who must quarantine for 14 days after returning home.

Vietnamese Prime Minister Nguyen Xuan Phuc on Monday told police to crack down on illegal immigration. Vietnam has been closed to foreigners since March 22, but it shares land borders with China, Laos, and Cambodia, sections of which are hard to police because of jungles or mountainous terrain.

In April, authorities in China said they were stepping up border control measures to limit the possibility of imported infections. One expert told Reuters at the time that China’s land border with Southeast Asian countries was “too long and porous to be enforced properly.” The border areas have a history of drug smuggling and human trafficking owing to the lack of strict controls.

Police arrested a Chinese man in Da Nang for allegedly heading a group that helps people illegally cross the border from China to Vietnam, Vietnamese state media reported on Sunday. However, Vietnamese authorities have not said the local cases in Da Nang are linked to illegal border crossings.

In June and early July, Vietnam launched a “travel bubble” for business travelers from Japan, and it was considering a similar arrangement with Taiwan. Thailand had named Vietnam as one of the countries it was thinking of including in its tourism bubble. The weekend’s spate of new local cases could put those plans in jeopardy. Should it grow, the outbreak could hit Vietnam’s economy, which has been one of the Asia’s few bright spots.

Fragile recoveries

Vietnam’s emerging crisis mirrors the regression of neighbors in the region that also seemed to have the virus under control.

Hong Kong, for example, is enduring an ongoing third wave of coronavirus infections, which are likely the result of quarantine exemptions. More than 40% of the city’s total cases, which now stand at 2,634, have been recorded this month, as have the majority of its 20 deaths.

Hong Kong requires all incoming travelers to quarantine for 14 days on arrival, but it’s been exempting airline crews, sea crews, and truckers driving in from mainland China. The government suspended some of these exemptions on Sunday, but some health experts said the tightening came too late.

Mainland China is also seeing a flare-up. It reported 57 local cases on Sunday, its highest daily increase since April. The cases are located in its far-western Xinjiang province and in the northeast provinces of Liaoning and Jilin, which border North Korea.

Australia is experiencing a second wave of infections that’s been linked to security staff working in hotels where returning travelers were quarantining. Staff monitoring those in quarantine failed to follow infection control measures and then passed the coronavirus onto their families and friends, sparking a community outbreak.

Like Vietnam, Hong Kong, mainland China, and Australia were considering starting travel bubbles with other regions before their new waves of cases. The relapse in Vietnam—once a model for outbreak control—is a signal to the rest of the world that reopening, even domestically, holds the risk of further outbreaks.