Researchers who mixed together bird flu and ordinary flu viruses created three extremely virulent new strains, a reminder that influenza viruses can swap genes to create dangerous offspring.
Their experiment, reported in the Proceedings of the National Academy of Sciences, demonstrates that as H1N1 swine flu wanes, other forms of flu continue to circulate and could make surprise appearances.
A World Health Organization experts committee was meeting on Tuesday to try to decide whether the H1N1 swine flu pandemic has peaked. But experts agree H1N1 might change or come back in a different form or recombine with another flu strain.
And H5N1 avian influenza is still circulating. It has infected 478 people and killed 286 of them since it re-emerged in Asia in 2003.
That particular strain especially frightens flu experts because it is so deadly when it does infect humans. They worry that H5N1 could either mutate or re-assort with another flu strain to become more easily able to spread among people.
Yoshihiro Kawaoka of the University of Wisconsin-Madison and colleagues made mixtures of H5N1 and the common circulating H3N2 strain.
Researchers still do not understand all of the factors that make flu so capable of causing disease, so virulent and easily transmitted. So Kawaoka's team used trial-and-error to swap out genes, checking if it would be possible to make a flu strain with H5N1's deadly properties and H3N2's ability to pass from one person to another.
They generated 254 different new strains and found that a mutation from an H3N2 strain first isolated in Tokyo gave the virus an ability to easily infect.
Tests in mice showed 22 strains were more pathogenic than the seasonal H3N2 virus - meaning they could cause disease more efficiently. Three were also more virulent - meaning they killed more easily - in the mice they tested.
"Our data demonstrate that reassortment between an avian H5N1 virus with low pathogenicity in mice and a human virus could result in highly pathogenic viruses," the researchers wrote. "Our findings highlight the importance of surveillance programs to monitor the emergence of human H5 reassortant viruses."
They said companies and governments need to continue to develop vaccines against all circulating influenza strains, including H5N1, and to encourage people to be vaccinated.
Flu viruses are especially error-prone, meaning that they mutate easily. They change gradually as they pass around the world but can also make sudden changes.
H1N1 has been circulating on and off since the 1918 pandemic but made a big change last year to create the swine flu pandemic. Experts believe it came from pigs.