China's financial hub of Shanghai, with a population of around 25 million, has conducted multiple rounds of mass nucleic acid testing as it strives to contain the worst COVID-19 resurgence in the country in two years.

Zhao Jing, an expert on the city's COVID-19 work team, explained the necessity in an interview with China Media Group on Wednesday.

A nucleic acid test detects genetic traces of the novel coronavirus in samples taken from people to determine whether a person is infected or not.

"The precondition for conducting mass nucleic acid test is that we know there are infections in communities, but not sure who are infected and in which community," he said.

"In that case, large-scale or even sweeping nucleic acid testing must be carried out. The goal is to quickly identify the infected from the general public, so as to control the source of infection," he added.

Shanghai reported 2,634 symptomatic cases on Wednesday, up from 2,494 a day earlier. Asymptomatic cases stood at 15,861, down from 16,407 the day before.

There were 441 new cases reported outside quarantined facilities, up from 390 a day earlier, which means community transmission of the coronavirus was still occurring.

Why are multiple rounds of screenings needed? Zhao gave two reasons.

First, because of the incubation period of the virus, one round of test may not be able to detect all the infected.

Second, although nucleic acid test is regarded as the gold standard for identifying infections, it's not 100 percent accurate.

Therefore, round after round of tests are needed, until all infections in communities are identified and quarantined, he said.

For hundreds of years, the basic principles to contain infectious diseases have remained unchanged, which is to control the source of infection, cut off the route of transmission, and protect the susceptible groups of people, the expert noted.