Vietnam’s capital will conduct mass testing for 21,000 people who recently returned from the central city of Da Nang, Hanoi’s ruling body said on Thursday, as authorities race to contain the first domestic outbreak of novel coronavirus disease (COVID-19) in the country since April.

The mass testing will take place from Thursday to Saturday, using rapid test kits, the city’s administration said in a statement.

Hanoi on Thursday registered its first case of COVID-19 linked to the Da Nang outbreak and has ordered bars to shut and has banned large gatherings from midnight on Wednesday.

The capital’s only two community-based COVID-19 cases as of Thursday morning both recently came back from trips to Da Nang.

Hanoi Center for Disease Control director Khong Minh Tuan said over 21,000 people in Hanoi have returned from Da Nang since July 8.

Very few of them — those exhibiting symptoms of the respiratory disease — had been tested for the coronavirus as of Wednesday evening, Tuan said.

Health experts said it is almost certain that more community cases will be detected in Hanoi once the mass testing is conducted.

“What needs to be done without delay is advising returnees from Da Nang [since July 8] to stay inside their homes and wait to be tested,” a health expert said.

Vietnam has recorded 43 community-based cases, all traced back to Da Nang, since Saturday last week, after having gone 99 days without a single local transmission, according to a Tuoi Tre News count.

Thirty-four of the patients are in Da Nang, three in Quang Nam, one in Quang Ngai Province, two in Hanoi, two in Ho Chi Minh City, and one in Dak Lak.

Authorities are conducting aggressive contact tracing to stall the virus.

The Southeast Asian country’s COVID-19 tally sits at 459, with 369 having recovered as of Thursday morning.

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