Zhongshan city, Guangdong province, has immediately started an emergency response plan including a large-scale nucleic acid testing campaign after a local COVID-19 asymptomatic carrier surnamed Guo was detected on Thursday.

Meanwhile a total of 435 close contacts of Guo have now been put under further medical observation, according to a statement released by the city’s health commission late Thursday.

Guo, 23, an employee of a local company, was diagnosed as an asymptomatic carrier after her nucleic acid test result came out positive again on Thursday.

Guo returned to the Zhuhai airport from Nanjing, capital of East China's Jiangsu province, on Monday, and took a train back to Zhongshan and rode back to her dormitory that evening via a car hailed online.

Guo had a negative nucleic acid test result on July 14 when she left Zhongshan for Nanjing, where a minor outbreak of COVID-19 is underway now. She visited the cities of Nanjing and Wuxi.

Guo went to work in her company after returning to Zhongshan. Her nucleic acid test yielded a positive result during routine testing on Wednesday, and she was later recorded as an asymptomatic carrier when her nucleic acid test came out positive again on Thursday.

Guo has now been transferred to a designated hospital for further medical observation and treatment.