Serologic survey of influenza A viruses subtype H7N9, H9, H5, H1N1pdm09, H3 and H1 in rural area of Deqing, Zhejiang

Objective To understand the seroprevalence of influenza A viruses subtype H7N9, H9 and H5, influenza A (H1N1) pdm09 virus and seasonal influenza A viruses subtype H1 and H3 in rural area of Deqing county, Zhejiang province, and the correlation among the infections of different viruses. Methods A total of 1228 serum specimens, including 725 collected before the H7N9 epidemic in 2013 and 503 collected after the H7N9 epidemic, were tested for the antibody against subtype H7N9, H9 and H5. Meanwhile, 398 of the 503 serum specimens collected after the H7N9 epidemic were also tested for the antibody against subtype H1N1pdm09 and subtype H1 and H3. Serum antibody were tested using hemagglutination-inhibition (HI) and considered as positive with HI titer 1:20 for subtype H7N9, H9, H5 and HI titer 1:40 for subtype H1N1pdm09 and subtype H3, H1. The seroprevalence and geometric mean titers (GMTs) were calculated, and the correlation was analyzed with Paired 2 test. Results Among 14 close contacts of 2 H7N9 infection cases, 2 were H7N9 virus positive (14.3%). Before the epidemic, no antibody against H7N9 virus was detected in general population (0/725). After the epidemic, 1 latent infection cases was detected (1/503). The rates of protective antibody against subtype H7N9, H9, H1N1pdm09, H1 and H3 were 0.2% (1/503), 4.19% (22/503), 4.77% (19/398), 5.28% (21/398) and 11.31% (45/398), respectively, no antibody against subtype H5 was detected. No significant differences were observed in seroprevalence among different age groups. The results from the correlation analysis indicated that people infected with subtype H1N1pdm09 tended to have antibody against subtype H9 (r=0.108, P 0.05) and people who were positive for antibodies against subtype H3, H1 were more likely to be positive for subtype H1N1pdm09 (r=0.504, P 0.05; r=0.111, P 0.05). Conclusion Our results showed that the general population in rural area in Deqing had no immunity to H7N9, H5 viruses and the seroprevalence of subtype H9, H1N1pdm09, H3 and H1 were still low, indicating that the epidemic might occur in the future. Subtype H1N1pdm09 is considered to correlated with subtype H9, suggesting the possible cross-reaction of antibody response and common exposure.