Human infection with the H5N1 or H9N2 avian influenza virus has been reported in the city of Guangzhou in southern China.1,2 To assess the risk of avian influenza virus infection among humans, we conducted a serologic surveillance study in Guangzhou. A total of 2191 healthy persons were recruited from 230 workplaces and seven types of occupations from March 2007 through July 2008. This study was approved by the ethics committee of the Guangzhou Center for Disease Control and Prevention, and written informed consent was obtained from the subjects.

Serum samples obtained from these subjects were assayed for antibodies against H5 and H9 avian influenza virus with a hemagglutination-inhibition assay containing 4 hemagglutination units of inactivated H5N2 virus strain A/Turkey/England/N28/1973 and H9N2 virus strain A/Chicken/Shandong/6/1996. The results were further confirmed, as described previously,2,3 by means of a neutralization assay against 100 tissue-culture infective doses of H5N1 strains A/Hong Kong/486/1997 and A/Vietnam/1194/2004 and H9N2 strain HK/2108/2003.

The prevalence of anti-H5 antibodies was 0.2% and the prevalence of anti-H9 antibodies was 4.5% (Table 1). All anti-H5–positive persons were poultry retailers or wholesalers, but anti-H9–positive persons were identified in all study groups in this cohort. It has been reported that H5N1 and H9N2 viruses were cocirculating among poultry and humans in markets.4 Our results show that the positive rate of anti-H5 antibodies was much lower than that of anti-H9 antibodies among poultry retailers (0.8% vs. 15.5%, P<0.001) and among wholesalers (0.8% vs. 6.6%, P=0.001). This difference may be attributed to the wide use of H5 vaccine in poultry, whereas vaccination against H9 is not available. In addition, H9 avian influenza virus infection in poultry is usually asymptomatic and associated with virus shedding. Given that reassortant of H9N2 with other subtypes of avian influenza virus has been reported to occur in southern China,5 our finding highlights the potential risk of H9 avian influenza virus to public health.