Seasonal flu may have descended from deadly 1918 influenza pandemic - The Natural History Museum

What was discovered about the 1918 influenza virus?

After recovering a complete genome and two partial sequences from the three samples, the researchers looked at how the Berlin samples compared to a complete sequence of the virus isolated in Alaska during the disease's second wave. 

They found that most of the genetic differences were in the polymerase genes, which control the replication of a virus, and not the haemagglutinin genes which allow it to bind to a cell. 

'It was a surprise that the virus haemagglutinin was conserved and that most of the changes took place in the polymerase enzyme,' Thorsten says. 'When we assessed these changes, we found the Alaskan virus was up to twice as active as the Berlin variant, which may be related to the virus' adaptive processes as it spread through the human population.

'After the pandemic, there were years where there was still increased mortality but it is difficult to distinguish whether this was from an increase in immunity or a decrease in virulence. 

'From the reconstruction, it suggests that this was a substantially more virulent virus with significant differences from seasonal flu. However, whether a gradual decline or a sudden drop was responsible for the change is impossible to tell from our current data.'

Instead, the researchers were able to investigate how samples of different 1918 influenza viruses were related. They found that European and American samples were distributed among different branches of the virus' family tree, suggesting that the virus was circulating internationally.

The different waves that spread these viruses were found not to have been caused by different variants of the virus replacing each other, as has occurred in the COVID-19 pandemic, but repeatedly by the same lineage of influenza.

The evolution of the virus was further investigated using a molecular clock model, which is used to assess the timeline of evolution. The scientists found that the eight segments of RNA which make up the seasonal influenza virus' genome could have directly descended from the pandemic virus.

However, they couldn't rule out that seasonal virus may have resulted from the process of viral reassortment, where a host being infected by more than one virus at a time can lead to the production of a virus which contains genetics from both.

The scientists also regard their findings as provisional due to the low sample size of 1918 influenza genetic sequences. In future, they hope to identify more specimens to learn more about this pandemic, in the hope it can give us more information about how future pandemics could develop.