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Case One: Influenza Type A(H5N1)
Preliminary Serology Results
Friday, December 26, 1997 9 p.m. EST
Saturday, December 27, 1997, 10 a.m. local Hong Kong time
Contact: Media Relations Division 404-639-3286
BACKGROUND
The first reported case of avian influenza A(H5N1)occurred in May 1997 in Hong Kong, and
was laboratory confirmed in August 1997. The case involved a three-year-old boy who died from
complications related to the disease.
Following laboratory confirmation, the Hong Kong Department of Health invited CDC
scientists to assist in investigating the case. The Hong Kong Department of Health, staff of
the Queen Elizabeth Hospital, researchers at the Chinese University and the Hong Kong
University, and CDC collaborated on a large serologic survey --- collecting approximately 2000
blood samples. Samples were collected from the following groups: people in contact with the
boy, people potentially in direct contact with the virus such as lab workers, people in
contact with poultry, and a "control group" of people not associated with the case.
A total of 921 samples were tested at least twice, using a new micro-neutralization assay test
developed at CDC to measure antibody to the H5N1 virus. The new test was quickly developed
because the most commonly used influenza antibody test was not sensitive in detecting antibody
against the avian influenza A H5N1 virus. The test continues to be refined.
SURVEY RESULTS
A total of nine (less than 2 percent) of the 502 people potentially in contact with either
the child, the virus in the lab or poultry were antibody positive for influenza A(H5N1). None
of the 419 control group was antibody positive to A(H5N1).
Based on the preliminary antibody tests related to case one, people were more likely to
test positive for antibody to influenza A(H5N1) virus if they were poultry workers or exposed
to the virus in the lab. Poultry workers had the highest percentage of positive results.
However, in three of the cases, the possibility of direct exposure to virus in the laboratory
or high-level exposure to poultry was not clear, leaving open the possibility of human to
human transmission.
Antibody positive found in potential contact with the child
0 of 4 family members
1 of 54 health care workers
1 of 261 staff, students and their parents
1 of 63 neighbors
Antibody positive found in people who handled the virus (lab)
1 of 73 laboratory workers
Antibody positive found in people exposed to animals
5 of 29 poultry workers
0 of 18 pig farm workers or neighbors
DISCUSSION
People known to have extensive exposure to poultry or direct contact with the virus in
laboratories, clearly, were more likely to have antibody to influenza A(H5N1) virus.
In addition, based on these results, the question of human to human transmission of the
virus remains unclear for a variety of reasons. For example, the history of contact with
poultry was unknown for an elderly neighbor of the case patient who tested positive.
Similarly, a classmate of the case patient was also positive. The case-patient could have
transmitted the virus to the classmate, or the classmate could have been exposed to the same
primary source. However, for one of the persons who tested positive --- the health-care worker
--- no history of contact with the virus in a laboratory setting or high-level exposure to
poultry was apparent. If human to human transmission is occurring, these results suggest that
transmission is inefficient.
Several aspects of the survey limits interpretation of the results. First, the sample of
contacts was not randomly chosen. Second, because the need for results from the new antibody
test was urgent, scientists are still refining interpretation of results. Finally, the
epidemiology of this virus may be evolving.
Importantly, CDC and the Hong Kong Department of Health are continuing surveillance and
conducting larger studies related to the additional cases. As the data increases and are
analyzed, it may change conclusions drawn today from this limited survey.
Centers for Disease Control and Prevention
Office of Communication
Divison of Media Relations
Atlanta, GA
URL: http://www.cdc.gov/media/pressrel/serology.htm
Updated: Monday, December 29, 1997
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