Cold-chain food contamination is a likely cause of the recent COVID-19 resurgence in Beijing, according to a research paper by medical authorities and universities published in the National Science Review last week.

The paper said Beijing's June cluster of novel coronavirus infections probably originated in high-risk overseas areas. It added that cold-chain transportation could become the new route of transmission.

On June 11, a patient was found to be associated with the Xinfadi wholesale market, which sells fruit, meat and vegetables in Beijing. That case broke the city's 56-day string with no new confirmed local cases.

The authorities immediately launched a major round of coronavirus tests for people who had been to the market, as well as for food items sold there

Up to five salmon samples tested positive, including one sample whose unopened packaging was contaminated.