The COVID-19 outbreak in Beijing, which took place after 56 days without new cases, indicated that the novel coronavirus was brought from outside into Xinfadi wholesale market, the latest source of new cluster infections, said Wu Zunyou, chief epidemiologist at the Chinese Center for Disease Control and Prevention said during an interview with CCTV on Friday.

It is difficult to pin down the source, Wu said. The tracing process has been implemented from two aspects: people and goods transported to Xinfadi market.

Additional investigations into early cases are underway to address two questions, said Wu. The first is for those who worked in and visited Xinfadi market, how come some got infected while others did not. The second is to determine whether the virus was brought by people or by objects.

Coronavirus found on shrimp packaging

China's General Administration of Customs on Friday reported the detection of the novel coronavirus on packages and the inner wall of a container of frozen shrimps imported from Ecuador on July 3.

Wu regarded the news as a very important warning that it is possible for imported products to carry the virus into China, but further scientific proof is needed to see if it can cause new outbreaks.

Imported goods, especially those from countries hit by COVID-19 and those with frozen packaging and under cryopreservation, should be inspected more closely or even banned to reduce the risk, Wu said.

Significant mutation?

The main structure of the coronavirus has not changed, said Wu, adding it was very good news for vaccine research.

He said though the virus has small amino acid changes all the time, it does not alter the virus structure.