“It’s crucial to understand where this virus came from so we can understand how to stop future outbreaks in the future,” said Anne Rimoin, an infectious disease epidemiologist at UCLA.
To this end, on January 14, 2021, the World Health Organization sent a group of 17 international experts to Wuhan to work with Chinese researchers on a thorough investigation into the origin of the virus.
Researchers have long said that SARS-CoV-2 has a zoonotic origin, meaning that it probably jumped from animals to humans when humans came in contact with an animal infected with the virus. This contact may include handling the infected animal, eating it, or preparing the animal for marketing according to Rimoin.
However, experts did not know exactly how the virus had entered humans and reached a final conclusion about the origin of SARS-CoV-2 may take years. They also do not know where or when the virus first entered humans, and several studies suggest that it may have been present elsewhere in the world – perhaps circulating at low levels – before the great outbreak in Wuhan, China.
“You are trying to reconstruct events from a year and a half ago with incomplete sampling 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 there is any hint of previous studies of infectious diseases, the origin of the virus may remain secret. The best comparison is the 2003 SARS outbreak, which was caused by a close cousin of the virus causing COVID-19 and eventually traced back to a single population of horseshoe crab bites.
But the search took more than five years. “I think they were pretty lucky,” Vincent Racaniello, a professor of microbiology and immunology at Columbia University’s College of Physicians and Surgeons, said of the SARS study. “We still have not found the source of the Ebola virus outbreak after many years of looking,” he added. “It is not easy.”
The joint WHO-China report is seen as a first step in what is likely to be a years-long investigation that released its findings last week. But the report itself is riddled with controversy. Following the release, the United States and 13 other countries raised concerns about the report in a joint statement, arguing that the international inquiry was “significantly delayed and lacked access to complete, original data and samples.”
However, many experts say that the report, although incomplete, is an important first step.
The researchers examined four major theories about how the virus spread to humans, ranking those who ranked these theories by probability, from “highly probable” to “highly unlikely.”
The intermediate host theory: This theory suggests that the virus was transmitted from an original animal host to an intermediate host, such as mink, pangolins, rabbits, raccoon dogs, domestic cats, rivets or badgers, and then directly infected humans through live contact with the other animal.
WHO-China study conclusion: “probably very likely”
The zoonotic overflow theory: The zoonotic overflow theory suggests that SARS-CoV-2 was transmitted directly from an animal, probably a bat, to humans. This transmission could have occurred through agriculture, hunting or other close contact between humans and animals.
WHO-China study conclusion: “possible to probable”
The theory of the frozen food chain: The “cold chain” theory suggests that transmission of SARS-CoV-2 from animals to humans may have taken place through contaminated frozen food. A frozen food product contaminated with animal waste containing SARS-CoV-2 could have transmitted the virus to humans without direct human-animal contact.
WHO-China study conclusion: “possible”
Controversial laboratory leak theory found to be ‘highly unlikely’
As part of the study, researchers returned to the Huanan Seafood Market associated with the first known cluster of cases in Wuhan. They also visited Hubei Provincial Hospital for Integrated Chinese and Western Medicine, where some of the first COVID-19 cases were treated and looked at virus sequencing data. The viral sequencing showed that various smaller variants of SARS-CoV-2 spread in Wuhan in December 2020.
“This again suggests that the virus may have been circulating a little longer than people had realized,” said Dominic Dwyer, an epidemiologist and member of the WHO’s research team.
Viral sequencing also showed that the Huanan market was probably not the primary 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, according to the WHO-China report.
“The market was certainly an amplifier, but probably not the actual source of the whole outbreak,” Dwyer said.
Previous genomic sequencing showed that the virus originated naturally, and the WHO-China team rated the lab leak theory as “highly 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 news conference on the report’s findings, noting that he was ready to deploy additional missions with specialist experts to do so.
“Science cannot rule things out like that,” said Peter Daszak, a zoologist and member of the WHO’s research team, about laboratory density. “You can only really show positive findings, you can not prove a negative. But what we found is that the escape of the laboratory was highly unlikely.”
The report found that the most likely pathway 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 may include tracing the first cases of the virus; study of market vendors for unusual peaks in antibodies and examine locations with concentrations of animals we know are susceptible to SARS-CoV-2.
Rimoin hopes the pandemic has shown that disease surveillance is the key to preventing future outbreaks and not just responding to them. As population growth and climate change push humans 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.
ABC News’ Sasha Pezenik, Sony Salzman and Eric Silberman contributed to this report.
Eric Silberman, MD, a resident physician in internal medicine at Northwestern Memorial Hospital, is a contributor to the ABC News Medical Unit.