Volume 8, Number 2—February 2002 Lack of Evidence for Human-to-
Human Transmission of Avian Influenza A (H9N2) Viruses in Hong
Kong, China 19991 In April 1999, isolation of avian influenza A
(H9N2) viruses from humans was confirmed for the first time.
H9N2 viruses were isolated from nasopharyngeal aspirate
specimens collected from two children who were hospitalized with
uncomplicated, febrile, upper respiratory tract illnesses in
Hong Kong during March 1999. Novel influenza viruses have the
potential to initiate global pandemics if they are sufficiently
transmissible among humans. We conducted four retrospective
cohort studies of persons exposed to these two H9N2 patients to
assess whether human-to-human transmission of avian H9N2 viruses
had occurred. No serologic evidence of H9N2 infection was found
in family members or health-care workers who had close contact
with the H9N2-infected children, suggesting that these H9N2
viruses were not easily transmitted from person to person. In
April 1999, two World Healt