Seasonal influenza originates from South-East Asia

Medical Tribune June 2008 SF I

David Brill

Seasonal influenza epidemics appear to emerge in East and South East (E-SE) Asia before spreading actross the globe, according to new research which could lead to improvements in future vaccine selection.

Scientists from the World Health Organization’s (WHO) Global Influenza Surveillance
Network used a technique called antigenic cartography to analyze the hemagglutinin component of some 13,000 human influenza A (H3N2) viruses that had appeared across six continents from 2002 to 2007.

“We were able to identify different strains of H3N2 as they arrived at new locations around the world over the 5 year study period,” said Colin Russell of the University of Cambridge, UK – lead author of the study.

“And we found solid evidence that each year since 2002, H3N2 viruses had migrated out of what we call the East and South East Asian Circulation Network.”

Influenza viruses are thought to be responsible for some half-a-million deaths worldwide each year. The WHO convenes twice annually to decide which particular strains to put into the latest influenza vaccines, but it can take up to 12 months to manufacture the 300 million doses required.

The research, published in the journal Science, found that once viruses leave E-SE Asia they typically take around 6 to 9 months to appear in Europe and North America, and do not subsequently reappear in Asia. Studying viral evolution patterns in S-SE Asia could therefore help to forecast which strains are going to reach the northern hemisphere the following year –
facilitating the strain selection process and leading to more effective vaccines.

Associate Professor Leo Yee Sin, clinical director of the communicable disease centre
at Tan Tock Seng Hospital, Singapore, commented that: “The findings presented in this study are of great interest to the world.

“It is an interesting notion that the E-SE Asia region can serve to predict likely circulating
strains. It is important to monitor the trend of influenza in this region and to prospectively validate the findings of this study,” she added.

According to the researchers there is not one particular place in E-SE Asia where new influenza strains evolve. It appears rather that viruses continually circulate within this regional network, giving rise to overlapping epidemics – the timing of which are driven primarily by climatic factors.

Oceania appears to be the first stop for H3N2 viruses leaving E-SE Asia, followed by Europe and North America. Viruses do not typically reach South America for a further 6 to 9 months – a delay that could be explained by the lack of direct travel and trade routes with Asia, the researchers suggest.

Vincent Chow, associate professor in the department of microbiology at the National University of Singapore, called the study a “landmark document.”

“It is an excellent model for global influenza monitoring … and represents a scientifically
sound basis for determining health and vaccine policy decisions in the future,” he said.

“Having the highest concentration of mega cities, Asia is an ideal melting pot for influenza viruses to spread, replicate and mutate,” added Chow.