COVID ‘fuse’ may have been lit weeks or months before the Wuhan market ‘bomb’

Researchers investigating the origin of the pandemic suggest the first infected person may have caught the coronavirus from an animal source as early as October, 2019 — well before the late December 2019 outbreak at the market.

“That [seafood market in Wuhan] was definitely connected to a lot of the early cases, but it’s now clear that the epidemic started well before that,” said Canadian researcher Michael Worobey, professor and head of the University of Arizona’s Department of Ecology & Evolutionary Biology.

“Rather than being the source of early infections from animals to humans in that market, it was probably really a human-to-human amplifier of outbreak, Worobey told Quirks & Quarks host, Bob McDonald. 

The researchers used a combination of genetic techniques and epidemiological models to estimate when the SARS-CoV-2 virus first jumped from an animal to a human, and how long it might have spent being passed to just one or two people at a time before an explosive spreading event occurred. 

Their simulations also suggest that during this early period of a small number of infections — when the viral “fuse” was burning — it’s quite likely the chain of infection could easily have been broken, with the virus going extinct and no pandemic occurring. That happened nearly three-quarters of the time in the team’s simulations. 

Getting to the index case

Using viral genomes from individuals who were infected from late December 2019 to March 2020, Worobey and his colleagues used a “molecular clock” to look back in time.

The SARS-CoV-2 virus mutates on average once every two weeks, which allows the researchers to create a family tree of the viral genomes to trace back to the original ancestor. 

They discovered that the original ancestor of the coronaviruses that was the source of all the strains they found probably existed as early as mid-November 2019. But this was likely not the first human case.

When you drop a virus with the features of SARS-CoV-2 with the ability to transmit quite efficiently in humans into Wuhan, 70 per cent of the time it actually just goes extinct.– Prof. Michael Worobey, University of Arizona

“That ‘fuse’ analogy is really great because the common ancestor might take you back to the virus that was at the heart of the explosion of the outbreak, but there may have been many cases before that time point, including the index case that predates that common ancestor,” Worobey explained.

To figure out the length of the fuse, they combined the molecular clock and simulations of the epidemic, with known data about the virus and how it spread, to find the index case that is the first person to become infected in 2019. 

“By doing that, we see that the index case may have been back as far as mid-October and probably no later than mid-December,” he said.

A matter of luck

By running these simulations over and over again, they estimated that only about 10 people were infected with the virus as the fuse was burning for weeks or even months before the first superspreading bomb went off.  

The person who was first infected by an animal source may have only passed it onto one other person, who maybe passed it onto one or two others, and so on. Epidemiological evidence suggests that most of the time with a virus like SARS-CoV-2, this chain of transmission is likely to die off. It may have simply been bad luck that this didn’t occur.

“When you drop a virus with the features of SARS-CoV-2 with the ability to transmit quite efficiently in humans into Wuhan, 70 per cent of the time it actually just goes extinct. It sputters out of existence, on its own accord.”

Worobey said this was was initially a surprise.

From the virus’s perspective, getting into the seafood market in Wuhan was a lucky break, he said, allowing its spread to become amplified to large numbers of people, and making it much more difficult to control the outbreak.

“In some ways, the stars aligned in just the wrong way in this pandemic,” Worobey said.

Lesson to avoid future pandemics

The nature of COVID-19 infections also worked against us in this case.  Unlike the SARS and MERS outbreaks that were much more severe in terms of symptoms,  the mild or even asymptomatic nature of most cases probably kept it below the radar.

“That’s what is the sort of killer with this kind of outbreak,” said Worobey.

While Worobey has worked as an epidemiologist for many years, as a younger man he worked as a firefighter in British Columbia. He thinks the sophisticated surveillance system employed by fire prevention services provides a useful example and analogy for what we need to spot outbreaks like this in the future.  

“There are sensors all across the province that detect where lightning strikes. And then after a big lightning storm, the first thing in the morning, a plane goes up and tries to see where the smoke is so you can get to the fire before it becomes a huge conflagration,” he said. 

“We still don’t have the equivalent with potential pandemic viruses.” 

Produced and written by Sonya Buyting.