A new study raises concerns that it may be possible for airborne transmissible, human-to-human avian H5N1 flu viruses to evolve in nature.

The study looked at five mutations identified previously in the controversial bird flu studies published in the journals Nature and Science—led by Yoshihiro Kawaoka of the University of Wisconsin and Ron Fouchier of Erasmus Medical Center in Rotterdam, the Netherlands, respectively—which would make it possible for bird flu to spread from human to human.

In those studies, the researchers experimented with bird flu strains to show which mutations would be necessary for the virus to evolve to become transmissible between mammals.