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.
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.
