Twelve people in the eastern Chinese city of Qingdao have tested positive for COVID-19, municipal health authorities reported early Monday, days after a holiday week when hundreds of millions of people traveled throughout the country.

The latest outbreak, which includes six asymptomatic cases, is linked to a chest hospital in Qingdao responsible for treating passengers arriving from abroad who tested positive for COVID-19 upon entering China, the local health commission said Sunday.

Among the dozen cases are patients who visited the hospital for non-coronavirus treatments and their family members, as well as health aids working at the facility. The infection routes are still unclear.

Qingdao, which is located in the coastal Shandong province, last month reported that two port workers who had handled imported frozen seafood tested positive for COVID-19 but did not show symptoms. No additional infections were identified after testing over 210,000 people who had been in close contact with the two workers, or who were otherwise considered high-risk, according to the health authority Friday.

In response to the current outbreak, local authorities have begun citywide testing to screen all 9 million residents within five days. Meanwhile, the chest hospital has closed its outpatient clinic.

China has experienced several flare-ups of COVID-19 since the country’s initial outbreak subsided earlier this year. This summer, Beijing reported hundreds of infections linked to a massive food market, and the city subsequently sealed off several residential communities and shut down schools. Together with large-scale testing, such aggressive measures have also controlled outbreaks in the provincial-level regions of Jilin, Xinjiang, and Yunnan.

Health authorities across the country have issued advisories warning people against nonessential travel to Qingdao, according to Sixth Tone’s sister publication The Paper.

Last week was the country’s National Day holiday, which saw 637 million people traveling domestically. Over 4 million visited Qingdao. Shandong’s capital of Jinan has called for anyone who has visited Qingdao since Sept. 23 to get tested for the coronavirus.

Editor: Kevin Schoenmakers.

(Header image: Residents line up for nucleic acid tests in Qingdao, Shandong province, Oct. 12, 2020. People Visual)