A pandemic 2009 H1N1 flu (pH1N1) virus combined with an H5N1 avian flu strain produced a hybrid as virulent as the H5N1 parent, according to researchers at St. Jude Children's Research Hospital in Memphis, Tenn. In their study, published in the Journal of Virology, they used reverse genetics to create several pH1N1 (A/California/04/2009) viruses expressing individual genes from an H5N1 strain (A/Hong Kong/483/1997) and observed increased replication in one recombinant that expressed the hemagglutinin gene of HK/483.
Greater replication also corresponded to increased virulence in mice, similar to that of the parent H5N1 strain. The team also observed that serial passage of the hybrid virus through human lung epithelial cells "resulted in increased pathogenicity, suggesting that these viruses may easily adapt to humans and become more virulent." When the parent H5N1 strain was passed sequentially through these cells, in contrast, it grew weaker.http://jvi.asm.org/cgi/content/short/JVI.05582-11v1