LAUSR

Abstract Objectives In Korea, the National Immunization Program provided trivalent inactivated influenza vaccines (IIV3) to all children aged 6 – 59 months during the 2017-2018 season. In this study, we aimed to evaluate the vaccine effectiveness (VE) of IIV3 in children during the 2017-2018 season. Methods Children aged 6 -59 months who were tested for influenza for their acute respiratory illness were included in four hospitals during the 2017-2018 influenza season. We estimated the VE of IIV3 by test-negative case-control design based on the rapid influenza diagnostic test (RIDT) or reverse transcription polymerase chain reaction (RT-PCR) test results. Results A total of 4,738 children were included in this study. The number of laboratory-confirmed influenza cases was 845 (17.8%), and there were 478 cases of influenza A, 362 cases of influenza B. The adjusted VE based on RT-PCR was 53.4% (95% CI, 25.3-70.5) against any influenza, 68.8% (95% CI, 38.7-84.1) against influenza A, and 29.7% (95% CI, -35.1-61.8) for influenza B. The adjusted VE based on RIDT was 14.8% (95% CI, -4.4-30.0) against any influenza, 24.2% (95% CI, 3.1-40.2) against influenza A, and -5.1% (95% CI, -42.6-21.4) against influenza B. Age-specific VE based on RT-PCR against any influenza was 44.1% (95% CI, -0.2-67.8) in children aged 6 months to 2 years and 59.3% (95% CI, 8.8-81.9) in children aged 3 to 5 years. Conclusion Our results suggest moderate protection (53.4%) of IIV3 against RT-PCR laboratory-confirmed influenza in children in the 2017-2018 influenza season. However, the RIDT hampered the validity to assess VE during influenza season. Caution is needed when interpreting RIDT-based test negative design influenza VE study.