The avian H5N1 influenza virus outbreak in Hong Kong in 1997 proved to be highly virulent when first transmitted to humans, but the reasons for this virulence have remained unclear. In August 26 Nature Medicine, Sang Heui Seo and colleagues at St Jude Children's Research Hospital, Memphis, USA, show that lethal H5N1 influenza viruses, unlike other human, avian and swine influenza viruses, are resistant to the antiviral effects of interferons and tumor necrosis factor α (Nat Med 2002, DOI:10.1038/nm757).

Seo et al. observed that replication of human H5N1/97 influenza virus in St Jude porcine lung epithelial cells was unaffected by treatment with interferonα, γand TNF-α. In addition, they showed that the nonstructural (NS) gene of H5N1 viruses and the presence of glutamic acid at position 92 of the NS1 molecule is associated with this resistance. Pigs infected with recombinant human H1N1 influenza virus that carried the H5N1...