More than a year after a “mysterious pneumonia” that sickened workers in a fish market in China, scientists are still collecting clues as to where SARS-CoV-2 – the virus that causes COVID-19 – came from.
“It’s critical to understand where this virus comes from so we can understand how to stop future outbreaks in the future,” said Anne Rimoin, an infectious disease epidemiologist at UCLA.
The investigation into the origin of the virus is crucial for public health and science reasons, but it has also sparked tensions between world powers, especially between the United States and China, whose leaders accused each other of lack of transparency and xenophobia during the pandemic .
“It’s not about pointing the finger – it’s about understanding it so we know how to do it better in the future,” said Rimoin.
To this end, the World Health Organization sent a group of 17 international experts to Wuhan on January 14, 2021 to work with Chinese scientists on an in-depth investigation into the origin of the virus.
Scientists have long said that SARS-CoV-2 has zoonotic origins, meaning that it likely jumped from animals to humans if humans came into contact with an animal infected with the virus. According to Rimoin, this contact can consist of handling the infected animal, eating it or preparing the animal for the market.
However, experts weren’t quite sure how the virus got into humans, and it could take years to come to a definitive conclusion about the origin of SARS-CoV-2. They also don’t know where or when the virus first made its way to humans, and several studies suggest it may have been present elsewhere in the world – perhaps circulating at low levels – before the major outbreak in Wuhan, China.
“You’re trying to reconstruct events from a year and a half ago with incomplete samples and data,” said Dr. W. Ian Lipkin, director of Columbia University’s Center for Infection and Immunity, to ABC News. “We may never know exactly what happened.”
If previous infectious disease studies are any indication, the origin of the virus may remain in mystery. The best comparison is the 2003 SARS outbreak, which was caused by a close cousin of the virus that causes COVID-19 and was ultimately traced back to a single population of horseshoe crab bats.
But that search took more than five years. “I think they were very lucky,” said Vincent Racaniello, professor of microbiology and immunology at Columbia University’s College of Physicians and Surgeons, of the SARS study. “We have still not found the source of the Ebola virus outbreaks after many years of searching,” he added. “It is not easy.”
The WHO-China joint report is considered a first step in what will likely be years of investigation, which released its findings last week. But the report itself is mired in controversy. Following its release, the United States and 13 other countries expressed concerns about the report in a joint statement, arguing that the international investigation had “been significantly delayed and lacked access to complete, original data and samples.”
But many experts say the report, while imperfect, is an important first step.
The researchers examined four main theories about how the virus got to humans, and ranked those theories in order of probability, from “very likely” to “extremely unlikely.”
The intermediate gas theory: This theory holds that the virus was transferred from an original animal host to an intermediate host, such as mink, pangolins, rabbits, raccoon dogs, domestic cats, civets, or ferret badgers, and then directly infected humans through live contact with the second animal. .
Research conclusion WHO-China: “likely to very likely”
The zoonotic spillover theory: The zoonotic spillover theory suggests that SARS-CoV-2 was passed directly from an animal, most likely a bat, to humans. This transmission may have occurred through agriculture, hunting or other close contact between humans and animals.
Research conclusion WHO-China: “possible to likely”
The frozen food chain theory: The “cold chain” theory suggests that SARS-CoV-2 transmission from animals to humans may have occurred through contaminated frozen food. A frozen food product contaminated with animal waste containing SARS-CoV-2 could have transmitted the virus to humans without any direct live contact between humans and animals.
Conclusion of the WHO-China study: “possible”
Controversial lab leak theory turns out to be ‘extremely unlikely’
As part of the investigation, scientists returned to the Huanan Fish Market in connection with the first known cluster of cases in Wuhan. They also visited Hubei Provincial Hospital of Integrated Chinese and Western Medicine, where some of the first COVID-19 cases were treated, and looked at viral sequence data. That viral sequencing showed that several smaller variants of SARS-CoV-2 spread in Wuhan in December 2020.
“That again suggests that the virus may have been circulating for a little longer than people realized,” said Dominic Dwyer, an epidemiologist and member of the WHO research team.
Viral sequencing also showed that the Huanan market was probably not the main source of the outbreak. While many early cases were related to the market, a similar number of cases were related to other markets, or no markets at all, the WHO and China report found.
“The market was certainly an amplifier, but probably not the real source of the whole outbreak,” said Dwyer.
Previous genomic sequencing showed the virus to be of natural origin, and the WHO-China team called the lab leak theory “extremely unlikely.”
But Dr. Tedros Adhanom Ghebreyesus, WHO’s director general, said he did not think the team’s assessment of the theory was comprehensive enough.
Further data and studies are needed to reach more robust conclusions, Tedros said at a press conference on the report’s findings, noting that he was willing to deploy additional missions with specialist experts to do so.
“Science can’t rule out that sort of thing,” said Peter Daszak, a zoologist and member of the WHO research team, of the lab leak theory. “You can only show really positive findings, you can’t prove negative evidence. But what we did find is that the escape from the lab was extremely unlikely.”
The most likely path, the report found, was the first theory, that the virus went from a bat to an intermediate animal and then to humans. According to Daszak, the next steps for investigation could be to trace the first cases of the virus; investigating suppliers in the market for unusual spikes in antibodies; and examining sites with concentrations of animals known to be sensitive to SARS-CoV-2.
Rimoin hopes the pandemic has shown that disease surveillance is essential to prevent future outbreaks, not just respond to them. As population growth and climate change push people further into animal habitats, “we’ll see more viruses hopping from animals to humans and we’ll see more disease events,” Rimoin said.
“An infection anywhere could be an infection anywhere,” she said.
Sasha Pezenik, Sony Salzman and Eric Silberman from ABC News contributed to this report.
Eric Silberman, MD, a resident of internal medicine at Northwestern Memorial Hospital, is a contributor to the ABC News Medical Unit.