The exchange of genetic material between two closely related strains of the influenza A virus may have caused the 1947 and 1951 human flu epidemics, according to biologists.
The findings could help explain why some strains cause major pandemics and others lead to seasonal epidemics. Until now, it was believed that while reassortment – when human influenza viruses swap genes with influenza viruses that infect birds – causes severe pandemics, such as the ‘Spanish’ flu of 1918, the ‘Asian’ flu of 1957, and the ‘Hong Kong’ flu of 1968, while viral mutation leads to regular influenza epidemics.
But it has been a mystery why there are sometimes very severe epidemics – like the ones in 1947 and 1951 – that look and act like pandemics, even though no human-bird viral reassortment event occurred.