Seroprevalence to Avian Influenza A(H7N9) Virus Among Poultry Workers and the General Population in Southern China: A Longitudinal Study

Background. Confirmed cases of avian influenza A(H7N9) virus infection in humans continue to occur in mainland China. Few confirmed cases have occurred in poultry workers despite potentially higher rates of exposure. Methods. A serological survey was conducted in May and December 2013 in poultry market workers, and in March and September 2013 in the general population. Blood samples were collected and tested for antibodies to H7N9 and H5N1 viruses by hemagglutination inhibition (HI) assays. Multivariable analysis was employed to identify risk factors related to H7N9 infection indicated by serology among poultry workers. Results. In the poultry workers, 36 of 501 (7.2%) in May and 56 of 375 (14.9%) in December had HI antibody titers >= 1:160 to H7N9. Of 96 individuals who participated in both surveys, 52 (54.2%) workers had a >= 4-fold rise in H7N9 antibody titers from May to December. In a multivariable analysis, female sex (odds ratio [OR], 2.713; 95% confidence interval [CI], 1.098-6.705) and >= 10 years of occupational exposure (OR, 3.592; 95% CI, 1.246-10.354) were identified as risk factors for infection. Seroprevalence against H5N1 at >= 1:160 was low in May (4/501 [0.8%]) and December (3/375 [0.8%]). In the general population, 0 of 417 individuals in March and 0 of 408 individuals in September had antibody titers >= 1:160 to H7N9 or to H5N1. Conclusions. Although none of the participants in our study had virologically confirmed H7N9 infection, the high proportion of poultry workers with serologic evidence of H7N9 infection between May and December 2013 suggests a substantial risk of mild H7N9 infections in this group, supporting stricter control measures in live poultry markets.