Most pandemic viruses that have jumped from animals to humans do not show signs of special adaptations to infect people, except the 1977 influenza outbreak, which was likely “sparked by a laboratory strain”, according to a new study.
Until now, such zoonotic viruses, which jump to humans from wildlife, have been assumed to first acquire mutations that enable sustained human-to-human spread.
But the new research upends these assumptions, revealing that most viruses were already “pre-adapted” for humans before their outbreaks.
Measurable changes in the virus genome typically seem to appear only after they spillover to humans and begin sustained transmission in people.
“This work has direct relevance to the ongoing controversy around Covid-19 origins,” said Joel Wertheim, an author of the study published in the journal Cell.
“From an evolutionary perspective, we find no evidence that SARS-CoV-2 was shaped by selection in a laboratory or prolonged evolution in an intermediate host prior to its emergence,” said Dr Wertheim, an evolutionary biologist from the University of California, San Diego School of Medicine.
The absence of this evidence is indicative of the Covid-19 pandemic originating from a natural zoonotic event, scientists say.
“It represents another nail in the coffin for theories invoking laboratory manipulation,” Dr Wertheim said.
In the study, researchers analysed viral genomes from outbreaks caused by influenza A virus, Ebola virus, Marburg virus, mpox virus, Sars-CoV as well as the Sars-CoV-2 virus, which was behind the Covid-19 pandemic.
They particularly looked at virus genomes from an evolutionary period immediately preceding human outbreaks, where any substantial adaptation before jumping to humans is likely to leave an imprint.
Scientists did not find any evolutionary signal in the genomes of most viruses that would suggest they were being “pre-adapted” for humans before outbreaks.
“Rather than requiring rare, finely tuned adaptations in animals, many viruses may already possess the basic capacity to infect and transmit between humans,” Dr Wertheim said.
“If a virus had been extensively passaged in a lab before an outbreak, we would expect to see it in the evolutionary record. In nearly all pandemics we’ve studied, that signal simply isn’t there,” he said.
However, there was “one historical outlier,” researchers say.
Unlike other pandemic viruses, the 1977 H1N1 influenza, dubbed the “Russian flu”, reemerged almost unchanged after a 20-year absence.
This is a pattern difficult to reconcile with natural evolution, scientists say.
The viral outbreak was first reported by the Soviet Union in 1977 and lasted until 1979.
Researchers say the 1977 H1N1 strain showed a clear shift consistent with viruses that propagated in cell culture or in laboratory animals.
“Our results provide new molecular evidence supporting the long-suspected idea that the H1N1 pandemic was sparked by a laboratory strain – possibly in the context of a failed vaccine trial,” Dr Wertheim said.
The research, according to scientists, provides a benchmark for distinguishing natural outbreaks from scenarios involving laboratory handling.
“Our goal is not just to understand the past, but to be better prepared for the future,” the evolutionary biologist said.
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