Scientists have discovered that influenza epidemics from the most common subtype come from viruses originating in East and Southeast Asia.
Cambridge University researchers examined 13,000 samples of influenza A (H3N2) virus collected through the World Health Organization Global Influenza Surveillance Network from 2002-2007.
New strains of the virus emerged in East and Southeast Asia, travelled across to Europe and North America six to nine months later, and then on to South America, says the paper in Science Journal. They did not usually return to their ‘countries of origin’.
Flu viruses tend to cause disease outbreaks in the winter months of temperate countries and in the rainy seasons of tropical ones. As the rainy season occurs at different times across East and Southeast Asia, this, and the wintertime epidemics in temperate areas, allows the virus to circulate all year round in this region.
This enables these strains to be the source of epidemics that occur in other parts of the planet, the investigators found.