Serologic assay for avian-origin influenza A (H7N9) virus in adults of Shanghai, Guangzhou and Yunnan, China

• Sera were collected in winter of 2011 from adults of Shanghai, Guangzhou and Yunnan.
• Sera were assayed for anti-HA and neutralizing antibodies to influenza H7N9 virus.
• Low levels of protective immunity to H7N9 pre-existed in adults of the three areas.
• Pre-existing immunity in Guangzhou seems stronger than that in Shanghai and Yunnan.

Background and objectiveTo investigate serologic status for novel avian influenza A (H7N9) virus in different areas in China, we examined serum samples collected in winter of 2011 from adult population of Shanghai (eastern China), Guangzhou (southern China) and Yunnan (southwest China) for the antibody responses to this virus.Study designA total of 900 stored serum samples of adult outpatients (300 samples for each area) were subjected to anti-hemagglutinin (HA) antibodies assay using enzyme-linked immunosorbent assay (ELISA) and neutralizing antibodies assay using H7N9 pseudotyped particles (H7N9pp) and authentic H7N9 virus based neutralization test.ResultsAnti-H7 antibodies were detected in 164, 186 and 123 samples from three areas above, respectively. Among anti-H7 positive sera, 20, 42 and 13 samples had neutralizing titers of ≥10 when 8 × 102 focus-forming units (FFU) of H7N9 pseudotyped particles (pp) were adopted in neutralizing assay, respectively. When neutralizing antibodies were assayed using classic microneutralization (MN) test, MN titers of ≥10 were found in 7 samples from Guangzhou, but none from Shanghai and Yunnan.ConclusionLow levels of protective immunity pre-existed in some general adult population of the three areas, and pre-existing immunity against H7N9 in Guangzhou appears stronger than that in Shanghai and Yunnan.