Two H9N2 viruses were isolated, for the first time, from humans in Hong Kon
g in 1999. Isolation Of influenza viruses with a novel subtype of the hemag
glutinin (HA) drew attention of health care authorities worldwide from the
view of pandemic preparedness. Sequence analysis of the HA genes reveals th
at HA of A/Hong Kong/1073/99 (H9N2) is most closely related to that of A/qu
ail/HK/G1/97 (H9N2) that contains the internal genes similar to those of Ho
ng Kong/97 (H5N1) viruses. Phylogenetic and antigenic analyses demonstrated
the diversity among H9 HA. A/Hong Kong/1073/99 was shown to cause a respir
atory infection in Syrian hamsters, suggesting that the virus can replicate
efficiently in mammalian hosts. We developed a whole virion test vaccine w
ith a formalin-inactivated egg-grown HK1073. Intraperitoneal administration
of the vaccine twice to hamsters conferred a complete protection against c
hallenge infection by the MDCK cell-grown homologous virus. Receptor specif
icity of HK1073 appeared different from that of other avian influenza virus
es of H9 subtype which recognize preferentially alpha -2,3 linked sialic ac
id. Hemagglutination of HK1073 with guinea pig erythrocytes was inhibited b
y both alpha -2,3 and alpha -2,6 linked sialic acid containing polymers. Th
ese data suggested that HK1073 had acquired a broader host range, including
humans. Together with data so far available, the present study suggested t
hat isolation of the H9 influenza viruses from humans requires precaution a
gainst the emergence of a novel human influenza. (C) 2001 Elsevier Science
Ltd. All rights reserved.