The Covid-19 pandemic will end, but the virus that caused it — SARS-CoV-2 — isn't going away.
At this stage of the pandemic, it's unclear how the virus will affect humanity over the course of the coming decades. Part of the answer could lie in dozens of glass jars located in basements across Europe. These jars contain lung specimens from people who contracted influenza during the early 20th century.
In a paper published Tuesday in the peer-reviewed journal Nature Communications, a team of researchers reveals that H1N1 — a seasonal strain of the flu that sparked a much smaller pandemic in 2009 and 2010 — is likely a direct descendent of the virus that caused the historic pandemic in 1918.
"The subsequent seasonal flu virus that went on circulating after the  pandemic might well have directly evolved from the pandemic virus," study co-author Sébastien Calvignac-Spencer said at a press conference on Monday.
Samples from the 1918 pandemic are hard to come by
Living through a respiratory pandemic more than a century ago was a very different experience from what people alive today have seen during Covid-19. As many as 100 million people across the world died, but researchers barely understood what was going on. Many suspected that a virus was causing the illness, but that wasn't proven until the 1930s. And it wasn't until the 1990s that researchers could say for sure that the H1N1 subtype of influenza A drove the pandemic.
Very few samples from the pandemic have survived, and only a fraction of them have been preserved well enough to enable researchers to look closely at the remnants of the virus's genetic code. However, new methods are giving researchers the power to learn from samples that couldn't be used just a few years ago.
Researchers already had complete genomes from the viruses that killed two people in 1919. One of them died in New York and the other died in Alaska. In this new study, the researchers analyzed lungs that have been stored at the Berlin Museum of Medical History and at the Museum of Natural History in Vienna, Austria. They used recent methods for extracting and analyzing genetic material, eventually producing two partial genomes and one complete genome of the virus that brought the world to its knees in 1918.
The genome shines new light on important questions
The new sequences add important new data to the conversation about the 1918 pandemic, but it's still not a lot of information for researchers to work with. "We have to remain humble and consider all our results as provisional," Calvignac-Spencer says. "Our results, in a nutshell, show that there was genomic variation during that pandemic, too," he says. The 1918 pandemic occurred in waves, similar to the way Covid-19 unfolded.
Surprisingly, the variants that emerged during the 1918 pandemic don't appear to have competed with each other the way we've seen during Covid-19. "There's not any evidence for lineage replacement between the waves like we see with the different SARS-CoV-2 variants displacing one another," Calvignac-Spencer says.
Finally, the new genomes disagree with the prevailing hypothesis about what happened to the 1918 virus after the pandemic subsided. "We uncovered with the sequences and new statistical models [that] the subsequent seasonal flu virus that went on circulating after the pandemic might well have directly evolved from the pandemic virus entirely," Calvignac-Spencer says. They found evidence for such evolution in all eight segments of the viral genome.
Does that prove that SARS-CoV-2 will join the other coronaviruses and circulate through the human population for the next 100 years? There's no guarantee, but this research shows that something like that scenario has probably happened before.