Apr 16, 2008 (CIDRAP News) – Seasonal influenza viruses flow out of overlapping epidemics in East and Southeast Asia, then trickle around the globe before dying off, researchers from the World Health Organization (WHO) committee that selects the viral strains for the annual flu vaccine said today.
At a press conference, researchers said the findings, released ahead of print today in the Apr 18 issue of Science, could influence how experts pick the flu strains that will be included in each year's vaccine. Derek Smith of the University of Cambridge in the United Kingdom told reporters that identifying the source of the viruses allows global health officials to better predict which viruses are most likely to cause the most disease over the next year.
"We now know that East and Southeast Asia is where we should be paying the most attention," he said. The findings will likely pave the way for greater collaboration on flu virus surveillance in those regions, he added.
"Flu science and flu public health have to go hand and hand, because it [the influenza virus] is a very complicated pathogen," he said.
The group analyzed 13,000 samples of influenza A/H3N2 virus that were collected across six continents from 2002 to 2007 by the WHO's Global Influenza Surveillance Network. They said they chose that influenza subtype because it is currently the major cause of flu-related illness and deaths. They compared differences in hemagglutinin (HA), a surface protein, among the different samples.
In 10% of the samples, the researchers also compared the sequences of the gene that codes for HA. The two analyses enabled the researchers to identify different strains of H3N2 as they arrived at new sites over the 5-year period.
The results revealed that newly emerging strains of H3N2 appeared in East and Southeast Asian countries about 6 to 9 months earlier than anywhere else. The strains generally reached Australia and New Zealand next, followed by North America and Europe. The new variants typically reached South America after an additional 6 to 9 months, the group reported.
Though the findings didn't suggest that any particular Asian country was a frequent source of the new strains, researchers reported that influenza viruses in Thailand, Malaysia, and Japan were antigenically less advanced than those in the rest of the region, implying that those countries were less likely to produce new strains.
Once the strains leave East and Southeast Asia, they enter an "evolutionary graveyard," the authors said in a press release today from the American Association for the Advancement of Science (AAAS), the publisher of Science. Older flu strains that leave North America, for example, aren't likely to infect people in Asian countries again, because populations would be immune to them, and the viruses would become extinct, Colin Russell, also from the University of Cambridge, told reporters.
The new findings on the travel routes of seasonal flu viruses cast doubt on other migration theories, Smith said. Some experts have suspected that the viruses migrate between the northern and southern hemispheres, come from tropical areas, or originate in China.
It's unclear why new variants appear in Asian countries first, but Russell said one contributing factor could be continually overlapping influenza epidemics in Asian countries. "It's like runners passing a baton, and evolution is occurring in that context," he said.
Though countries East Asia aren't very far apart, many have different climates and rainy seasons that occur at different times of the year. For example, Smith said Bangkok and Kuala Lumpur are only 700 miles apart, but their flu seasons occur at different times of the year. He also said Asian countries have intensive contact through air travel, which could also contribute to viral movement patterns.
The next step is to reach back into the WHO's surveillance database to analyze H3N2 viruses from the years before 2002 to see how robust the circulation pattern findings are, Smith said.
When journalists asked if the findings had any bearing on likely circulation patterns for pandemic influenza strains, Russell said the findings apply only to seasonal flu viruses that were included in the study. "It [seasonal flu] is completely distinct," he said.
Russell CA, Jones TC, Barr IG, et al. The global circulation of seasonal influenza A (H3N2) viruses. Science 2008 Apr 18;320(5874):340-6
Apr 16 Science press release