EFFICIENT EPIDEMIOLOGICAL SURVEY
On Dec. 9, the city reported a locally transmitted COVID-19 case, a staffer engaged in disinfection at a quarantined hotel for inbound flights. Many locally transmitted COVID-19 cases have emerged since then.
The viral genome sequencing of 137 locally transmitted COVID-19 cases in the outbreak identified the strains as the highly infectious Delta variants, which are highly homologous with imported cases of an inbound flight on Dec. 4, said Liu Feng, director of the provincial center for disease control and prevention.
Residents queue up for nucleic acid test at a testing site in Xi'an, northwest China's Shaanxi Province, Dec. 21, 2021. (Xinhua/Li Yibo)
"Many positive cases didn't show obvious symptoms in the initial stage. Therefore, they tended to ignore their physical condition, which led to the rapid spread of the virus inadvertently," said Wei Xiaoli, deputy director of the city's disease control and prevention center.
Epidemiological investigation found that the trajectories of the positive cases are complex and involve a wider area, Wei said.