The 1918-19 flu pandemic killed between 17 million and 100 million people worldwide. But while some virus genomes have been sequenced and it is now known to be an H1N1 flu, there is still a lot of mystery surrounding it. Viral and genetic material normally survives very poorly, leaving many unknowns.
Now, an analysis of newly discovered samples suggests that one of our seasonal flu strains may be a direct descendant of the virus that caused the 1918 pandemic.
An international team of researchers discovered the flu in lung samples from 1918 and 1919, held in the museum’s archives in Berlin. Sébastien Calvignac-Spencer, a researcher at the Robert Koch Institute in Germany and co-author of an article describing the research, published in nature Communications, says the team was investigating lung specimens kept in the basement of a local museum, to see if they could extract any information about respiratory viruses that the owners had.
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“Honestly, I wasn’t very hopeful. I was wrong because it worked beautifully from day one. We were quickly able to assemble the complete genome and collect genome-wide information for two other specimens,” says Calvignac-Spencer.
“The 1918 Spanish flu is still a big mystery,” says co-author Thorsten Wolff, also at the Robert Koch Institute.
“So when Sébastien contacted me, saying, ‘Oh, we just found some virus relics, more or less in our backyard,’ I was completely excited and interested in examining these genomes.”
The researchers found that the genomes differed significantly from other previously reconstructed genomes from the 1918 flu. There were also some important changes between the older and newer versions of the virus, both likely making the flu better at evading the human immune system.
As the specimens come from before the peak of the pandemic — and then during its peak — they provided insights into how the virus evolved. This evidence implies that the modern seasonal H1N1 flu is a descendant of the 1918 flu, having become much less virulent as it evolved.
The samples also show that – like COVID-19 – the 1918 flu evolved into new strains and spread across the world.
“There was genomic variation during this pandemic as well,” says Calvignac-Spencer.
“And when we interpreted that variation, we detected a clear signal of frequent transcontinental scattering. It’s not too surprising in these times, but it’s good to show.
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Not all pandemics are the same. In the historical samples, “we also show that there is no evidence of lineage replacement between waves, as we see today with two different variants of SARS-CoV-2 that replaced each other.
However, it does offer data on the global possibilities of a pandemic. “Another thing we found with the sequences and new statistical models is that the subsequent seasonal virus that continued to circulate after the pandemic may have evolved directly from the pandemic virus entirely,” he says.
This contradicts current most popular hypotheses about the origins of H1N1. Calvignac-Spencer cautions that all their results are tentative, based on small samples as they are.
“A major limitation of this type of research is that we only have a very limited sample. So we have to be humble.”