By IANS,
Washington : Seasonal influenza strains constantly mutate in overlapping epidemics in Asia and sweep the rest of the world every year, according to a study.
By focussing surveillance efforts on East and Southeast Asia, it may be possible to forecast which flu strains are most likely to cause epidemics and help experts decide which strains should go in the flu vaccine each year.
Annual influenza epidemics are believed to cause three to five million cases of severe illness and between 250,000 and half million death yearly, according to WHO.
Colin Russell of the University of Cambridge and colleagues analysed 13,000 samples of influenza A (H3N2) virus, collected across six continents from 2002 to 2007 by WHO’s Global Influenza Surveillance Network. This subtype of influenza is currently the major cause of flu-related illness and death in humans.
Together, these analyses allowed the researchers to identify different strains of A (H3N2) as they arrived at new locations around the world over the five-year period.
A (H3N2) is a subtype of the influenza A virus, and it is one of the three flu viruses included – in dead or in a weakened state – in the flu vaccine.
The others are the influenza A (H1N1) subtype and the influenza B virus. Each year, the WHO decides which strains within these three categories to include in the next vaccine, based on the recent activity of strains that are currently in circulation.
“This study advances our knowledge of how new flu strains spread across the globe and how epidemics arise,” said Katrina Kelner, Science’s deputy managing editor.
The authors emphasised that the flu vaccine works extremely well, protecting about 300 million people each year, and that people should continue to be vaccinated annually. But, from time to time, a new strain begins infecting people after the vaccine has already been produced.
“Flu epidemics appear to be driven by seasonal factors such as winter, or rainy seasons. So there can be cities that are only 700 miles away from each other, such as Bangkok and Kuala Lumpur, which have epidemics six months apart.
The study, by a team of researchers from Europe, Australia, Japan and the US, will appear on Friday issue of Science.