Wu Fan, a member of the team of public health experts involved in Shanghai's COVID-19 prevention and control efforts, said disease control professionals worked for 15 consecutive hours on Jan 20 and 21.

Based on the logistics worker's contact history, they conducted in-depth analysis to define a virus transmission chain.

"They analyzed the possible extent of the chain, the public areas involved and the number of people who might have been infected," Wu said.

She said the team then investigated links in the transmission chain. More than 30 close contacts of the patient were identified and placed under hotel quarantine that night, and 2,817 people who were in close contact with those quarantined were traced.

Wu said, "Community workers in different areas of the city raced against time to knock on people's doors before dawn, telling them not to leave home to prevent the possibility of the virus spreading."

Disease prevention workers checked areas frequented by those confirmed with COVID-19, including their workplaces, over the past 14 days.

They checked individuals' routes to determine whether their activities in different locations could have contaminated these areas or spread the virus to others.

They took into account whether those infected were wearing a face mask, the time they spent in a particular area, and whether they touched anything or spoke to anybody.