资源环境科技发展态势分析平台

Science’s COVID-19 reporting is supported by the Pulitzer Center.

Beijing’s confirmation of a COVID-19 case on 11 June ended a run of 55 days without reported local transmission. Since then, the outbreak has burgeoned and the city has responded with fierce determination to rein it in. As of today, it has tested 356,000 people, confirming 137 cases, according to a news report by Xinhua, the state-owned news agency. The city has locked down some residential compounds, closed all schools, and canceled hundreds of flights.

Virtually all of the infections have been linked to a massive wholesale food market that has been temporarily shuttered. The link to the market has triggered comparisons to the seafood market in Wuhan that played a role at the early stages of the pandemic, and speculation that the virus arrived in fish imported from Europe. But the real source of the outbreak is still a mystery.

Beijing reported its last case of local COVID-19 transmission in mid-April. The current outbreak began when a man with no history of recent travel visited a doctor on 10 June with a fever and chills. He tested positive for SARS-CoV-2, the virus that causes COVID-19, and was hospitalized the following day. Officials think he or a close contact was infected at the Xinfadi Agricultural Wholesale Market, a massive 112-hectare complex housing 2000 stalls selling produce, seafood, and meat with 10,000 customers and workers visiting daily, according to Xinhua. This led to the massive effort to test market workers, customers, and even residents of nearby neighborhoods.

Authorities have reported that a number of surfaces in the market tested positive for SARS-CoV-2, including a cutting board in a booth handling imported salmon—a finding that has drawn lots of attention in the local media. There is no evidence coronaviruses infect fish, but one hypothesis is that infected workers in Europe contaminated the fish or its packaging during processing. Genomic sequencing shows the viral variant behind the new outbreak is related to strains China has found in people returning from Europe, according to a China Daily report that quotes Yang Peng, an official with the Beijing Center for Disease Prevention and Control. In addition to seafood, Yang suggested imported meat as a possible source. But he acknowledged that a market employee or visitor may have picked up the virus elsewhere and simply spread it to other people at the market. “Where exactly the virus came from is still uncertain,” Yang  said.

Dirk Pfeiffer, a veterinary epidemiologist at the City University of Hong Kong, doubts the virus arrived at the market in fish: “I think it is much more likely, and therefore plausible, that it was brought to the market by infected humans.” If the contaminated seafood hypothesis is true, other places handling European salmon should have seen outbreaks, adds epidemiologist Keiji Fukuda of the University of Hong Kong. The genomic sequence, which has not been made public yet, could offer more clues, he says.

Six months ago, many of the earliest COVID-19 cases were linked to another market, the Huanan Seafood Wholesale Market in Wuhan, China. Such markets host dozens to hundreds of independent operators offering a range of meat, seafood, and produce, and sometimes live wild game. Many scientists believe SARS-CoV-2 likely originated in bats and may have passed through an “intermediate host” before jumping to humans—perhaps at the Wuhan market.

But Pfeiffer says the meat and live animals sold at the markets may not be the only reason viruses spread there. The sheer volume of people passing through and working in the markets and the “suboptimal hygienic conditions inherently represent an increased risk for amplification of virus.” The humid, chilled air at markets may provide an environment in which viruses thrive.

The virus’ return to Beijing is “another cautionary tale to not take anything about COVID for granted,” Fukuda says. With the vast majority of the world population susceptible to infection and the virus still circulating, “it is possible for any country, including [those] that have made major efforts to reduce transmission, to experience an outbreak” he says.