Previous studies have found that influenza virus vaccine effectiveness (VE) varies by season, circulating strain, age and geographic location. However, there is a lack of studies among hospitalised children, particularly in Europe and the Middle East. The authors estimated VE against influenza hospitalisation among children aged 6 months to 8 years in Israel in the 2015-2016, 2016-2017 and 2017-2018 influenza seasons using a test-negative design. 326 influenza-positive cases and 2821 influenza-negative controls were included (140 cases and 971 controls from 2015-2016, 36 and 1069 from 2016-2017, and 150 and 781 from 2017-2018). Over all seasons, VE was 53.9% for full vaccination (95%CI 38.6%–68.3%) and 25.6% for partial vaccination (95%CI -3% to 47%). In 2015-16, most viruses were influenza A(H1N1) and vaccine lineage-mismatched influenza B/Victoria; with the VE for fully vaccinated children being 80.7% (95%CI 40.3%–96.1) for influenza A virus and 23.0% (95%CI -38.5 to 59.4% for influenza B. During 2016-2017, influenza A(H3N2) predominated and the VE was 70.8% (95%CI 17.4%–92.4%). In 2017-2018, influenza A(H3N2), H1N1 and lineage-mismatched influenza B/Yamagata co-circulated, with a VE of 63.0% (95%CI 24.2%–83.7%) for influenza B and 46.3% (95%CI -7.2% to 75.3% for influenza A. Therefore influenza virus vaccination is effective in preventing hospitalisations among fully vaccinated Israeli children over three influenza seasons but not among partially vaccinated children. There was some cross-lineage protection in a season where the vaccine contained B/Victoria and the circulating strain was B/Yamagata but not in a season with the opposite vaccine-circulating strain distribution.
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