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Travel patterns of seasonal flu

18 April, 2008  

Outbreaks of the most common type of influenza virus, A (H3N2), are seeded by viruses that originate in East and Southeast Asia and migrate around the world, new research has found. This discovery may help to further improve flu vaccines and make the evolution of the virus more predictable.
Scientists at the University of Cambridge, in collaboration with scientists from the World Health Organisation (WHO) Global Influenza Surveillance Network, found that each year since 2002 influenza A (H3N2) viruses have migrated out of what the authors call the “East and Southeast Asian circulation network” (which includes tropical, subtropical, and temperate countries) and spread throughout the world.  Their findings are reported in the current edition of Science.

Annual influenza epidemics are thought to infect 5-15% of the world population each year, cause 3 to 5 million cases of severe illness, and between 250,000 and 500,000 deaths, according to the World Health Organization. The flu vaccine protects the 300 million people vaccinated each year. 

Because the flu virus evolves so quickly, there are a number of challenges involved in making the vaccine.  In order to create an effective vaccine, each year in February and September a WHO committee meets to select the strains of flu to use in the influenza virus vaccine.  These scientists (many of whom are co-authors on this study) decide which strains pose the greatest threat for the next flu season.  

One of the serious challenges to creating flu vaccines is that the global migration pattern of influenza viruses has been a mystery.  Several competing hypotheses have emerged including migration between the Northern and Southern hemispheres following the seasons, migration out of the tropics where influenza viruses were thought to circulate continuously, and migration out of China.

Colin Russell of the University of Cambridge and his colleagues analyzed 13,000 samples of influenza A (H3N2) virus, collected worldwide by the World Health Organization Global Influenza Surveillance Network between 2002 and 2007.
The analyses allowed the researchers to identify different strains of A (H3N2), the subtype of seasonal flu that causes the most disease, as they arrived at new locations around the world over the five-year period. The results revealed that new strains emerge in East and Southeast Asia and then about six to nine months later reach Europe and North America. Several months later still, the strains arrive in South America. Once viruses leave East and Southeast Asia they rarely return and thus regions outside of East and Southeast Asia are essentially the evolutionary graveyards of influenza viruses.
For reasons that aren’t well understood, flu epidemics typically occur during the winter months in the temperate regions of the northern and southern hemisphere and in tropical countries, flu epidemics often coincide with the rainy season. Because there is variation in the timing of the rainy season in different parts of East and Southeast Asia, combined with the wintertime epidemics in the temperate parts of the region, the overlap in the timing of epidemics gives the opportunity for influenza viruses to circulate year round in East and Southeast Asia. The authors find that this year-round circulation allows East and Southeast Asia to serve as the source of influenza A (H3N2) viruses for epidemics in the rest of the world.
WHO Global Influenza Surveillance Network