The 1918-19 influenza pandemic killed between 17 and 100 million people worldwide. But although some virus genomes have been sequenced and it is now known to be H1N1 influenza, there is still a lot of mystery surrounding it. Viral and genetic material generally survive very poorly, leaving many unknowns.
Now, an analysis of newly discovered samples suggests that one of our seasonal influenza strains may be a direct descendant of the virus that caused the 1918 pandemic.
An international team of researchers discovered influenza in lung samples from 1918 and 1919, preserved in museum archives in Berlin. Sébastien Calvignac-Spencer, a researcher at the Robert Koch Institute in Germany and co-author of a paper describing the research, published in nature Communications, He says the team was investigating lung specimens in the basement of a local museum, to see if they could extract any information about the respiratory viruses the owners had had.
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“Honestly, I didn’t have high hopes. I was wrong, because it worked wonderfully from day one. We were quickly able to assemble the entire genome and collect genome-wide information for two other specimens,” says Calvignac-Spencer.
“The Spanish flu of 1918 is still a big mystery,” says co-author Thorsten Wolff, also of the Robert Koch Institute.
“So when Sébastien contacted me and said, ‘Oh, we just found some relics of the virus, more or less in our backyard,’ I was completely excited and interested to see these genomes.”
The researchers found that the genomes differed significantly from previously reconstructed 1918 influenza genomes. There were also a couple of key changes between the older and younger versions of the virus, both of which likely made the flu better for evade the human immune system.
Because the samples come from before the peak of the pandemic, and then during its apex, they have provided insight into how the virus evolved. This evidence implies that modern seasonal H1N1 flu is a descendant of the 1918 flu, and became much less virulent as it evolved.
The samples also show that, like COVID-19, the 1918 flu evolved into new strains and spread around the world.
“There was also genomic variation during that pandemic,” says Calvignac-Spencer.
“And when we interpret this variation, we detect a clear signal of frequent transcontinental dispersal. Not very surprising these days, but that’s good for 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 SARS-CoV-2 variants replacing each other.
However, it offers data on the global possibilities of a pandemic. “Another thing we discovered with the sequences and the new statistical models is that the subsequent seasonal virus that continued to circulate after the pandemic might well have directly evolved from the pandemic virus altogether,” he says.
This contradicts the most popular current hypotheses about the origins of H1N1. Calvignac-Spencer cautions that all of her results are tentative, as they are based on small samples.
“A major limitation of this type of research is that we only have a very limited sample. So we have to remain humble.”