Some of the seasonal flu viruses that people can catch each winter may be a direct descendant of the strain behind the catastrophic 1918 "Spanish flu" pandemic, according to a new study in the journal Nature Communications.

Scientists from the Robert Koch Institute recently analyzed 13 lung specimens from different individuals collected in Germany between 1900 and 1931 and managed to get their hands on two partial genomes and one complete genome of the H1N1 influenza A virus, all from 1918.

The viruses come from victims of the 1918 influenza pandemic, often known by the misnomer "Spanish flu," one of the most severe pandemics in recent history. Wiping out up to 100 million people, the outbreak was the first of three flu pandemics caused by the H1N1 influenza A virus, followed by the 1977 Russian flu pandemic and the 2009 swine flu pandemic.

Not all H1N1 outbreaks are devastating on a global scale, however. Some strains of H1N1 influenza are endemic in humans and cause a small fraction of all influenza-like illnesses we see each winter around the globe. Contrary to other studies, this latest research hypothesizes that the seasonal outbreaks of H1N1 we see each year are caused by viruses that are direct decedents of those behind the infamous influenza pandemic of 1918.

The genetic material of a virus from over 100 years ago is notoriously tricky for scientists to work with, but the team was fortunate enough to closely analyze this century-old virus and gain insights into how it caused havoc across the globe – and still potentially lurks around to this very day. 

"The 1918 pandemic affected over half of mankind and killed 50 to 100 million people, but when we started this work there were only 18 specimens from which sequences were available and only two complete genomes, most from the US,” Sébastien Calvignac-Spencer, author of the study from Robert Koch Institute, said at a media conference. 

Just as you might expect, their analysis clearly suggested that the 1918 flu virus was spreading both locally and across continents. 

Crucially, the new genomes from 1918 Germany are from the earlier phases of the pandemic compared to other specimens, showing that the 1918 flu pandemic saw numerous waves much like the COVID-19 pandemic. Unlike COVID-19, they didn’t find any evidence of new variants of the 1918 flu virus rising, dominating, and replacing one another with each wave. However, it appears the 1918 flu virus did evolve over time to become better adapted to infecting humans.

Using molecular clock modeling, a method that allows evolutionary timescales to be estimated, they found that all genomic segments of the seasonal H1N1 flu – one of the many strains that can circulate each winter – could be directly descended from the initial 1918 pandemic strain.

There are no genetic sequences to show what happened to the virus over the course of the 1920s, but by the early 1930s, the H1N1 virus had become less virulent, effectively transitioning from a pandemic virus to a seasonal virus. According to the new study, H1N1 did this without reassortment, the exchange of genomic segments between different viruses, so seasonal flare-ups of the virus can be considered a pure descent of the pandemic strain.

The researchers stressed that any comparisons between the COVID-19 pandemic and the 1918 flu pandemic should be made with caution – these are very different viruses and very different conditions for the virus to spread. That said, some parallels can be picked out. 

Most scientists now believe that SARS-CoV-2, the virus that causes COVID-19, will become a seasonal endemic virus, perhaps no more severe than a common cold. After all, some suspect that the virus behind the 1889–1890 pandemic, often referred to as the "Asiatic flu" or "Russian flu," was a predecessor of human coronavirus OC43, which mainly causes mild respiratory symptoms nowadays. 

Perhaps, others have suggested, a meek and mild future could also be the fate of SARS-CoV-2.