A disease control expert in Shanghai said that a team comprising more than 3,000 disease control and prevention workers has been racing against time since last Thursday to trace the source of this new wave of infection in the city.

"We still need further investigations to discover the truth before we can reach a conclusion of who patient zero is," Pan Hao, deputy head of Shanghai's frontline COVID-19 working team with the Shanghai Municipal Center for Disease Control and Prevention, said on Monday.

A new infection case was reported in Shanghai on Sunday, bringing the total number of recent local transmission cases in the city to 13. All the cases were found through contact tracing of the confirmed patients.

"For a confirmed case, we'll try to restore the individual's history of activity and find out the venues that he or she has been to and his or her contact history with people and articles. We'll also take into consideration whether they have had contact with recovered COVID-19 patients," he said.

Wu Jinglei, director of the Shanghai Municipal Health Commission, said at a municipal press briefing on Monday that Shanghai has been imposing a 14-day quarantine for recovered COVID-19 patients after they are discharged from hospital.

"After meeting the national standard of being discharged from hospital, all such individuals are transported with closed-loop management measures to hotels or their homes for mandatory quarantine for 14 days," Wu said.

"Their temperature and health condition are closely monitored during the two weeks and they need to test negative for the nucleic acid test twice before their quarantine is lifted."

Four weeks after being discharged from the hospital, these individuals should go to designated medical institutions for health checkups, he added.

"Chances are that the COVID-19 virus, of which both the toxicity and level of infectivity probably surpass the flu virus, will coexist with humans for the long term," Zhang Wenhong, leader of the Shanghai team of experts in the clinical treatment of novel coronavirus pneumonia cases, said during a recent interview with Wenhui Daily.

"Chances are that COVID-19 epidemics may take place in winter every year and vulnerable people will face infection risks."