New research reveals differences in how the four types of influenza that cause seasonal viruses spread and the rate at which each virus evolves are important for determining which flu virus is in circulation.

All four influenza viruses that cause seasonal flu in humans — influenza A viruses H3N2 and H1N1, and influenza B viruses Yamagata and Victoria — cause similar symptoms and evolve by similar mechanisms.

Previous research conducted by the University of Cambridge showed that H3N2 viruses circulate continuously in East and Southeast Asia throughout the year, spreading to the rest of the world to cause seasonal flu epidemics. Given the fundamental similarities between H3N2, H1N1 and B viruses it was thought that H1N1 and B viruses would also emerge from East and Southeast Asia to cause yearly epidemics worldwide.

However, the work published today in Nature shows that while H3N2 viruses die out between epidemics and new viruses emerge from East and Southeast Asia every year, H1N1 and B viruses frequently circulate continuously between epidemics worldwide, which gives rise to a huge diversity in these viruses.

Dr Ian Barr, acting director of WHO Collaborating Centre for Reference and Research on Influenza (WHO CC) and co-author said, “This work represents another piece in the complex puzzle of influenza virus circulation and human infections and provides insights that will help develop better influenza vaccines that match strains circulating in the community.”

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