Researchers at Ohio State University have traced the evolutionary history of the seasonal H1N1, which first infected humans during the 1918 pandemic. Primary author Daniel Janies says H1N1 is one of three seasonal influenza A viruses that commonly infect humans.
The others are H1N2 and H3N2.
Within H1N1, two strains of virus circulate in humans -- a seasonal form and the pandemic form of influenza known as swine flu, which has sickened millions and killed thousands of people since it first emerged in North America last spring, Janies says.
Over time, the H1N1 strain of seasonal influenza surviving around the world has developed mutations that have caused it to become resistant to oseltamivir-based agents, or Tamiflu.
"Something happened in 2008, when drug resistance took hold," Janies says in a statement. "It is a pretty good bet that whatever pressure is in the environment, excessive use of Tamiflu or something else that was driving seasonal influenza to become resistant to Tamiflu is also going to apply to pandemic influenza. We can see it happening already."
The findings are published in the International Journal of Health Geographics.