AS THE world fights to tackle the covid-19 pandemic, a mystery remains: how and when did the virus cross over into humans? Doubt has been cast on the idea that it happened in the Huanan Seafood Market in Wuhan, China, in December, and now researchers are trying to identify the real source of the infection. The hope is that this knowledge could help prevent future pandemics of other new coronaviruses.
According to a study of the first 41 people hospitalised with covid-19 published in January (The Lancet, ), the first case of covid-19 was a man who showed symptoms on 1 December 2019. Unlike the majority of early cases, he had no links to the Huanan Seafood Market.
Since then, no one has been able to confirm where he caught the virus, or if he was even the first person to contract it. Another January analysis, of the first 425 covid-19 cases, conducted by the Chinese Center for Disease Control and Prevention and China’s National Health Commission, placed the first confirmed case a week later, on 8 December.
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But subsequent evidence hints that the outbreak probably began before December. Viral genome analyses suggest that the virus jumped from animals to humans in November (The Lancet, ), but it could have happened as early as late September (Journal of Medical Virology, ).
This is consistent with the on Chinese government documents that suggested the earliest case of covid-19 may have been a 55-year-old person from Hubei province who seems to have contracted the virus on 17 November.
The first cases to be flagged, in December, were reported by Wuhan doctors using a surveillance protocol designed to pick up pneumonias with unknown causes. The system was set up after the 2002-2003 SARS outbreak, to detect new viruses.
Following this, 91av understands that early efforts by Chinese authorities to identify covid-19 cases focused only on people with viral pneumonia who had traceable links or contact with the Huanan market.
This focus on pneumonia may mean that many milder early cases were missed. By December, infections had probably already spread outside Wuhan. A study of six children who contracted the covid-19 virus identified a girl who developed symptoms on 2 January (NEJM, ). She and her family live in Yangxin, more than 150 kilometres from Wuhan. None of them had travelled outside the county for a month before she became ill, and the researchers weren’t able to identify how she became infected.
One explanation for this could be that the virus has jumped into humans from animals several times. Bats are thought to be the reservoir of the covid-19 virus, but Richard Kock at the Royal Veterinary College in London says catching it from a chance encounter with a bat is unlikely. A more probable scenario is that other animals may have acted as intermediaries, amplifying the virus and enabling it to infect multiple humans in a method of trial and error, he says.
“The risk of new viruses emerging has enormously accelerated. We have to get a grip on that”
The covid-19 virus appears to be able to infect a range of hosts – lab studies have found that it readily infects rhesus macaques and ferrets. Sunda pangolins have been suggested as an intermediate host because they harbour coronaviruses similar to the covid-19 virus. So far, genetic analysis has found viruses in pangolins that are a more than 90 per cent match for the covid-19 virus, but none similar enough to be the direct precursor (Current Biology, ).
The role the Huanan market may have played in enabling the virus to cross over into people is now uncertain. “The problem is that most samples in the Huanan Seafood Market have been destroyed,” says Shan-Lu Liu at the Ohio State University.
“It’s still a possibility that the environmental contamination in the seafood market was from infected humans who were working there, rather than from an animal source,” says Benjamin Cowling at the University of Hong Kong.
The market is also one of 400 in Wuhan. “If there was an amplifying population of animals that were then supplying 400 markets, plus directly to restaurants, [there’s] a huge capacity for [crossover] events to take place,” says Kock.
Now that human-to-human transmission has spread the virus worldwide, some believe the hunt for patient zero – the first person infected in the outbreak – is of relatively little importance. “At our stage of the epidemic, it’s not really the prime focus to know where it comes from,” says Julien Riou at the University of Bern, Switzerland.
But Kock says identifying the source of the outbreak is crucial, given that three coronaviruses – the SARS, MERS and covid-19 viruses – have all emerged since 2002. “In evolutionary terms, that’s in microseconds,” he says. “The risk of these things happening has enormously accelerated. We have to get a grip on that.”
Knowing more about the event that led to the covid-19 virus spreading to humans could help us figure out how to stop it from happening again, says Cowling.
