The seasonal human flu virus may have descended from the 1918 Spanish flu strain, new research suggests.
The findings are based on an analysis of samples collected in Europe during the 1918 pandemic, which was the deadliest respiratory pandemic of the 20th century and killed between 50 and 100 million people.
Researchers detected mutations in the make-up of the H1N1 virus – or swine flu – that may have helped it better adapt to human hosts.
The seasonal human flu virus may have descended from the 1918 Spanish flu strain, new research suggests
Researchers detected mutations in the make-up of the H1N1 virus – or swine flu – that may have helped it better adapt to human hosts
HOW THE SPANISH FLU EMERGED
Experts believe the Spanish flu virus arose shortly before 1918, when a human H1 virus, which they think had already been circulating in the human population since about 1900, picked up genetic material from a bird flu virus.
The Human influenza A virus usually sees higher mortality rates for infants and the elderly, but the pandemic virus caused extensive deaths in people ages 20 to 40, primarily from secondary bacterial infections, especially pneumonia.
Experts believe this was because many young adults born from about 1880 to 1900 were exposed during childhood to a H3N8 virus circulating in the population, which had surface proteins that were very different from those of the H1N1 virus.
In contrast, most individuals born earlier or later than 1880-1900 would have had better protection because they were more likely to have been exposed a virus variant more similar to the 1918 virus.
The international team from Robert Koch Institute, University of Leuven, Charite Berlin and many others revealed more details on the biology of H1N1, as well as evidence of its spread between continents.
Sebastien 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.
This included six samples collected in 1918 and 1919.
Researchers believe that genetic differences between the samples are consistent with a combination of local transmission and long-distance dispersal events.
They compared genomes from before and after the pandemic’s peak which indicate there is a variation in a specific gene associated with resistance to antiviral responses and could have enabled the virus’ adaptation to humans.
The authors also conducted molecular clock modelling, 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.
According to the researchers, this contradicts other hypotheses about how the seasonal flu emerged.
Dr Calvignac-Spencer said: ‘Our results in a nutshell show that there was genomic variation during that pandemic.
‘And when we interpret it, we detect a clear signal for frequent transcontinental dispersal.
The findings are based on an analysis of samples (pictured) collected in Europe during the 1918 pandemic, which was the deadliest respiratory pandemic of the 20th century and killed between 50 and 100 million people
Nurses are pictured caring for victims of the Spanish Flu in 1918 in Massachusetts as the virus spread around the world
Members of the Red Cross Motor Corps are pictured wearing masks as they carry a patient on a stretcher into their ambulance in Missouri in October 1918
‘We also show that there’s not any evidence for lineage replacement between the waves — like we see today with Sars-CoV-2 variants that replace one another.
‘And another thing that we uncovered with the sequences and new statistical models is that the subsequent seasonal flu virus that went on circulating after the pandemic might well have directly evolved from the pandemic virus entirely.’
The findings are published in Nature Communications.
WHAT WAS SPANISH FLU?
The 1918 flu pandemic was unusually deadly and the first of two involving the H1N1 influenza virus.
It infected 500 million people globally, more than one-third of the world’s population, including people on remote Pacific islands and in the Arctic.
It resulted in the deaths of an estimated three to five per cent of the world’s population, making it one of the deadliest natural disasters in human history.
Spanish Flu resulted in the deaths of an estimated three to five per cent of the world’s population, making it one of the deadliest natural disasters in human history. This image shows soldiers from Fort Riley, Kansas, ill with the virus
Within months it had killed three times as many as World War I and did it more quickly than any other illness in recorded history.
Most influenza outbreaks disproportionately kill juvenile, elderly, or already weakened patients. By contrast, the 1918 pandemic predominantly killed previously healthy young adults.
To maintain morale, wartime censors minimised early reports of illness and mortality in Germany, Britain, France, and the United States. However, newspapers were free to report the epidemic’s effects in Spain.
This created a false impression of Spain as being especially hard hit, leading to the pandemic’s nickname Spanish flu.
The close quarters and massive troop movements of World War I hastened the pandemic and probably both increased transmission and augmented mutation, researchers believe.
The true global mortality rate from the pandemic is not known, but an estimated 10 per cent to 20 per cent of those who were infected died. This would lead to a death toll of between 50 to 100 million people.