Bird-Flu Virus Develops Multiple Distinct Strains

Speaking at the International Conference on Emerging Infectious Diseases (ICEID) in Atlanta, Dr. Rebecca Garten of the CDC said that H5N1 avian flu has diversified into two genetic types. But it gets even more complicated. "Back in 2003, we only had one genetically distinct population of H5N1 with the potential to cause a human pandemic, now we have two," she's quoted in a Red Herring article.

All the H5N1 viruses which had infected humans came from a genetic subgroup called "Z." However, those from birds belonged to the Z subgroup, but also to subgroups labeled "V," "W" and "G."

The World Health Organization (WHO) has broken down the virus types even more finely. It classifies the Z subgroup into two clades, called 1 and 2.

Clade 1 viruses were responsible for all human H5N1 infections in Vietnam, Cambodia, and Thailand in 2003 and 2004. However, in 2005, a clade 2 virus infected people in Indonesia.

"It was the first time that clade 2 viruses infected people," CDC spokesperson Jennifer Morcone. "There are now two distinct groups of viruses what we are aware have pandemic potential," she added. "This is the first time that we’re presenting genetic details of the two clades."
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"We now know that a vaccine candidate for clade 1 does not protect against for clade 2," added Ms. Morcone.

The CDC is already running clinical trials for a clade 1 vaccine candidate. A clade 2 vaccine is much further behind in development, however.

MedPage Today has additional information.