More than a year after “mysterious pneumonia” sickened workers at a Chinese seafood market, scientists are still gathering clues as to where SARS-CoV-2 – the virus that causes COVID-19 – came from. .
“It’s essential to understand where this virus came from so we can understand how to stop future outbreaks,” said Anne Rimoin, an infectious disease epidemiologist at UCLA.
Investigating the origins of the virus is crucial for public health and science reasons, but it has also raised tensions between world powers, especially the United States and China, whose leaders have accused each other of lack of transparency and xenophobia during the pandemic.
“It’s not about pointing the finger – it’s just about understanding it, so we know how to do better in the future,” Rimoin said.
To this end, on January 14, 2021, the World Health Organization sent a group of 17 international experts to Wuhan to work with Chinese scientists on an in-depth investigation into the origins of the virus.
Scientists have long said that SARS-CoV-2 has zoonotic origins, which means it probably jumped from animals to humans when humans came in contact with an animal infected with the virus. This contact could include handling the infected animal, consuming it or preparing the animal for the market, according to Rimoin.
However, experts did not know exactly how the virus reached humans and reached a definitive conclusion about the origins of SARS-CoV-2 could take years. I also don’t know where or when the virus first made its way into humans, and several studies suggest that it may have been present in other parts of the world – perhaps circulating at low levels – before the Wuhan outbreak. China.
“Try to reconstruct the events of a year and a half ago with incomplete sampling and data,” Dr. W. Ian Lipkin, director of the Center for Infection and Immunity at Columbia University, told ABC News. “We may never know exactly what happened.”
If previous investigations of infectious diseases are a clue, the origins of the virus could remain shrouded in mystery. The best comparison is the 2003 outbreak of SARS, which was caused by a close cousin of the virus that causes COVID-19 and eventually targeted a single horseshoe bat population.
But the search lasted more than five years. “I think they were lucky,” said Vincent Racaniello, a professor of microbiology and immunology at Columbia University’s College of Physicians and Surgeons. “We have not yet found the source of the Ebola outbreaks after many years of research,” he added. “It’s not easy.”
The WHO-China joint report is considered a first step in what will likely be a years-long investigation and published its findings last week. But the report itself has been shrouded in controversy. Following its launch, the United States and 13 other countries expressed concern about the report in a joint statement, claiming that the international investigation was “significantly delayed and lacked access to complete data and original evidence”.
But many experts say the report, while imperfect, is an important first step.
Investigators explored four major theories about how the virus spread to humans, classifying those in order of probability, from “very likely” to “extremely unlikely.”
Intermediate host theory: This theory proposes that the virus was transmitted from an original animal host to an intermediate host, such as minks, pangolins, rabbits, raccoons, domestic cats, civets, or ferrets, and then directly infected humans through live contact with the second animal.
Conclusion of the WHO-China survey: “probably to very likely”
Zoonotic breaking theory: Zoonotic discharge theory suggests that SARS-CoV-2 was transmitted directly from an animal, most likely a bat, to humans. This transmission could have happened through agriculture, hunting or other close contact between humans and animals.
Conclusion of the WHO-China survey: “possible to probable”
Frozen food chain theory: The “cold chain” theory suggests that the transmission of SARS-CoV-2 from animals to humans could have occurred through contaminated frozen foods. 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 survey: “possible”
The controversial theory of laboratory leaks was considered “extremely unlikely”
As part of the investigation, the scientists returned to the Huanan Seafood Market, associated with the first known group 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 analyzed viral sequencing data. Viral sequencing showed that various minor variants of SARS-CoV-2 spread to Wuhan in December 2020.
“This again suggests that the virus may have circulated a little longer than humans realized,” said Dominic Dwyer, an epidemiologist and member of the WHO investigation team.
Viral sequencing also showed that Huanan Market was probably not the primary source of the outbreak. While many early cases were connected to the market, a similar number of cases were associated with other markets or had no markets at all, the WHO-China report found.
“The market was certainly an amplifier, but it probably wasn’t the real source of the whole outbreak,” Dwyer said.
Previous genomic sequencing has shown that the virus has natural origins, and the WHO-China team has described the theory of laboratory leaks as “extremely unlikely”.
But Dr Tedros Adhanom Ghebreyesus, WHO’s director general, said he did not think the team’s assessment of the theory was broad enough.
Additional data and studies will be needed to reach stronger conclusions, Tedros told a news conference about the report’s findings, noting that it is ready to conduct additional missions with specialist experts to do so.
“Science cannot rule things out like this,” said Peter Daszak, a zoologist and member of the WHO’s investigation team. “You can only show positive findings, you can’t prove it’s negative. But what I found is that escape to the lab was extremely unlikely.”
The most likely way, the report found, was the first theory that the virus passed from a bat to an intermediate animal and then to humans. According to Daszak, the next steps for the investigation could include tracing the first cases of the virus; investigating market suppliers for unusual increases in antibodies; and examination of locations with known animal concentrations are susceptible to SARS-CoV-2.
Rimoin hopes the pandemic has shown that disease surveillance is key to preventing future outbreaks, not just responding to them. As population growth and climate change push people further into animal habitats, “we will see more viruses jump from animals to humans and we will see more disease outbreaks,” Rimoin said.
“An infection anywhere is potentially an infection everywhere,” she said.
Sasha Pezenik, Sony Salzman and Eric Silberman of ABC News contributed to this report.
Eric Silberman, MD, a resident physician in internal medicine at Northwestern Memorial Hospital, contributes to the ABC News Medical Unit.