The World Health Organization was asked in May 2020 to help identify the animal source of the virus and how it spread to humans. It convened a team of 17 international scientists, including one based in the U.S., and this year they conducted a four-week joint mission with 17 researchers from China. Their findings were released in a joint report in late March. Other groups are also investigating, including an expert panel convened by the medical journal The Lancet called the Covid-19 Commission.
Debate about Covid’s emergence has coalesced around two competing ideas: a laboratory escape or a spillover from animals. Scientists involved in the WHO-led mission ranked four scenarios in order of likelihood:
LIKELY-TO-VERY LIKELY: the virus spilled over via an “intermediary” host species;
POSSIBLE-TO-LIKELY: the virus spilled over to humans directly from an animal reservoir;
POSSIBLE: the virus was introduced via the food chain by contaminated food or packaging;
EXTREMELY UNLIKELY: the virus emerged as a result of a laboratory-related accident.
What do we know so far?
Bats were the source of two coronaviruses that caused lethal outbreaks in people during the past two decades - severe acute respiratory syndrome (SARS) and Middle Eastern respiratory syndrome (MERS) - and the flying mammals are considered the probable reservoir host for SARS-CoV-2 as well. (A reservoir host is an animal that harbors a pathogen but isn’t sickened by it.) After SARS-CoV-2 emerged, Shi Zhengli, who studies bat-borne coronaviruses at the Wuhan Institute of Virology, identified three closely related viruses that had been collected during the previous 15 years. The closest, about 96% identical, was isolated from a species of horseshoe bat, Rhinolophus affinis, in the southern Chinese province of Yunnan in 2013. Some researchers have linked that particular virus, known as RaTG13, to a mineshaft in Mojiang county there, where six men contracted a pneumonia-like disease in 2012 that killed three of them. Spillovers of such viruses from bats to humans may cause an average of 400,000 infections a year in southern China and Southeast Asia, researchers said in a study released ahead of peer-review and publication in September.
What role did that food market in Wuhan play?
Researchers aren’t sure. It could have acted as a contamination source, an amplifier for human-to-human transmission, or both. Epidemiological data identify the Huanan seafood and fresh produce market in central Wuhan as an early and major epicenter of Covid cases. Two of the three earliest documented infections were directly linked to the sprawling, cavernous market, as were 28% of all cases reported in December 2019. Overall, 55% of cases during December 2019 had an exposure to either the Huanan or other markets in Wuhan, with these cases more prevalent in the first half of that month. Testing after the market was closed (it was shut down at 1 a.m. on Jan. 1, 2020) found widespread contamination of the floor, walls, chopping boards and cleaning tools compatible with the virus being transmitted from infected people to surfaces.
What about the lab theory?
After meetings with scientists in Wuhan, including discussions about their bio-containment and safety practices, the WHO-arranged mission found it “extremely unlikely” that SARS-CoV-2 originated in one of several labs in the city conducting research on coronaviruses, a large family that includes those that cause the common cold. Although accidents have been known to occur, sparking rare outbreaks, the scientists said there had been no reports of the virus existing before it was detected in Wuhan. Researchers who have analyzed its viral sequence have concluded that it doesn’t have the genetic signatures of a lab-engineered virus. Also labs in Wuhan tested their staff and students for evidence of SARS-CoV-2 infection and found no cases. Danielle Anderson, an Australian virologist who was working as recently as November 2019 in the Wuhan Institute of Virology’s BSL-4 lab - the first in mainland China equipped to handle the planet’s deadliest pathogens - said she saw nothing to suspect the virus spread from there.
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