Sales and immunogenicity of commercial vaccines to H9N2 low pathogenic avian influenza virus in Korea from 2007 to 2017

The present study was conducted to monitor sales activity and immunogenicity of commercial H9N2 vaccines produced in Korea from 2007 to 2017. Recorded sales of H9N2 vaccine were around 671 million doses, with 10 million doses sold in 2007, rising to a peak of 93 million doses in 2016, with a slight fall in 2017. Multivalent combined vaccines made up around 90% of all vaccine sales, and around 30% of all vaccines were distributed by regional governments for free. The regional vaccination rate was the highest in Gyeonggi and Chungnam, respectively with proportional to the population of layer and breeder chickens. There have been no cases of field infection since 2009. The mean antibody titer was 5.82 log₂ across the study period. Our results suggest that continuous genetic monitoring of H9N2 viruses circulating in the field and updating the vaccine seed strain periodically are necessary in order to control H9N2 outbreaks.