Epidemiology: Seasonal H1N1 flu may be descended from 1918 pandemic strain

The seasonal H1N1 flu virus may be a direct descendant of the 1918 influenza strain that caused a global flu pandemic, suggests a paper published in Nature Communications. The findings are based on the genomic analysis of samples that were collected in Europe during the 1918 pandemic.

The 1918 pandemic is estimated to have cost the lives 50 to 100 million people worldwide. Our understanding of its spread and timing is based on historical and medical records, which indicate its peak occurred in the autumn of 1918 and it continued through to winter 1919. However, it was only in the 1930s that it was confirmed to be of viral origin, while more recent research has since suggested the virus was an influenza A virus (IAV) of the H1N1 subtype. Genomic analysis of the 1918 virus is difficult due to the rarity of viral sequences from the time period.

Sébastien Calvignac-Spencer and colleagues analysed 13 lung specimens from different individuals stored in historical archives of museums in Germany and Austria, collected between 1901 and 1931, which included 6 samples collected in 1918 and 1919. From these they were able to sequence two partial genomes collected in Berlin in June 1918 and a complete genome collected in Munich in 1918. The authors suggest that the genomic diversity of the samples is consistent with a combination of local transmission and long-distance dispersal events. They compared genomes from before and after the pandemic’s peak and suggest there is a variation in the nucleoprotein gene which is associated with resistance to antiviral responses and could have enabled the virus’ adaptation to humans. The authors also conducted molecular clock modelling, a method which allows evolutionary timescales to be estimated, and suggest that all genomic segments of the seasonal H1N1 flu could be directly descended from the initial 1918 pandemic strain. This contradicts other hypotheses which suggest the seasonal virus emerged through reassortment (the exchange of genomic segments between different viruses).

The authors highlight that samples are still scarce but the insights gained here into the evolution and progress of the 1918 flu pandemic show the value of prospecting historical archives.