INTRODUCTION: ?Given that Influenza A (H9N2) subtype viruses are recognized as a pandemic threat, we evaluated the overall burden of H9N2 infections among avian-exposed human populations.

METHODS: ?We performed a systematic literature search in PubMed, AGRICOLA, and Cab Abstracts databases from 1997 to 2013. Studies reporting serological evidence of human Influenza A(H9N2) infection among avian-exposed populations were included. We used a WHO-recommended case definition for serological evidence of infection through hemagglutination inhibition (HI) and microneutralization (MN) assays. We calculated overall seroprevalance through a random effects meta-analysis model.

RESULTS: ?Seroprevalence figures reported by the studies ranged from 1% to 43% (median: 9%) by HI, which was not significantly different from the seroprevalence estimated through the WHO-recommended case definition (median: 1.3%, range:0.5%-42.6%). Reported seroprevalence by MN ranged from 0.6% to 9% (median: 2.7%), which was greater than the seroprevalence estimated through the WHO-recommended case definition (median: 0.3%, range: 0.1%-1.4%).

CONCLUSION: ?A small proportion of avian-exposed humans had evidence of H9N2 infection. As the virus has a near global distribution in poultry, it seems likely that present surveillance efforts are missing mild or asymptomatic infections among avian-exposed persons. It seems prudent to closely monitor avian-exposed populations for H9N2 infection to provide pre-pandemic warnings.