The H1N1 seasonal influenza virus may be a direct descendant of the influenza strain that caused the Spanish flu in 1918. Researchers from the Robert Koch Institute (RKI) published this in the journal Nature Communications. They genetically analyzed tissue samples from patients from that time. However, they cannot yet determine how the pathogen transforms into milder influenza viruses.
The Spanish flu of 1918 and 1919 is considered the worst pandemic of the 20th century. It is said that 50 to 100 million people died from it worldwide. However, it remains unclear how the influenza A virus evolved during the pandemic and what made it so deadly.
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Probably not from Spain
This is also due to the fact that tissue samples from this period are extremely rare. In 2005, US researchers sequenced the first complete genome of a virus strain from an Alaskan woman whose body was preserved in ice. Analysis at the time indicated that the virus jumped from birds to humans. The second complete genome sequencing was followed in 2013.
A team led by epidemiologist Sebastian Kalviniak Spencer of the RKI examined 13 European samples of lung tissue dating from 1900 to 1931 that were preserved in formalin at the Berlin Museum of Medical History and in the Vienna Histopathological Collection. Six of these samples were taken in 1918 and 1919. Through this, the researchers were able to sequence two partial genomes and one complete genome of the influenza A virus. A comparison with American samples showed genetic differences between pathogens of different waves.
According to the study, the genetic diversity of 1918 influenza viruses corresponds to a combination of local transmission and transcontinental spread. Today it is assumed that the disease originated in the American Midwest and then reached Europe with soldiers during World War I.
Virulence and immunity interact
At a press conference about the study, co-author and virologist Thorsten Wolff explained that the observed genetic differences can likely be traced back to the virus’s adaptations. While the genomes of samples from the early days of the pandemic were more similar to the original avian viruses, later samples showed mutations that made it easier for the flu virus to evade the human immune system. This would also explain why the first wave is more moderate than the two subsequent waves.
The RKI researchers also used a genetic method known as the molecular clock, which can be used to estimate evolutionary timescales. Modeling based on this suggested that all of the genomic segments of the seasonal H1N1 influenza circulating today could be directly descended from the 1918 pandemic strain. However, the evolution from there is so far inexplicable.
Wolf reports that although there have been winters with increased deaths from influenza and pneumonia after the Spanish flu, they have decreased: “It is a process in which it is difficult to determine whether this is due to increased immunity of the population or decreased virulence of the pathogen.”
Wolf says it’s hard to predict when and whether the pathogens circulating today could become deadly again. For example, there was another H1N1 pandemic in 2009 and 2010 with swine flu. However, the circulation of H1N1 viruses today and the available vaccinations against it also mean that the immune system of today’s people is not as naive toward the pathogen since 1918 as it was 100 years ago.