Abstract
BACKGROUND: We estimated influenza vaccine effectiveness (VE) in 2015-2016 season against medically attended, laboratory-confirmed influenza, when quadrivalent inactivated vaccine (IIV4) was first introduced in Japan, using test-negative case-control design. Influenza A(H1N1)pdm09 cocirculated with B/Yamagata and B/Victoria during the study period in Japan. METHOD: We based our case definition on two laboratory tests, real-time reverse transcription polymerase chain reaction (RT PCR), and virus isolation and compared VEs based on these tests. In addition, VE was evaluated by rapid diagnostic test (RDT). Nasopharyngeal swabs were collected from outpatients who visited clinics with influenza-like illness (ILIs) in Hokkaido, Niigata, Gunma and Nagasaki prefectures. RESULTS: Among 713 children and adults enrolled in this study, 578 were influenza positive by RT PCR including, 392 influenza A and 186 influenza B, while 135 were tested negative controls. The adjusted VE by RT PCR for all ages against any influenza was low protection of 36.0% (95% confidence interval [CI], 3.1% to 58.6%), for influenza A was 30.0% (95% CI: -10.0% to 55.5%), and influenza B was moderate 50.2% (95% CI: 13.3% to 71.4%). Adjusted VE for virus isolation for A(H1N1)pdm09 was 37.1% (95% CI: 1.7% to 59.7%), Yamagata lineage 51.3% (95% CI: 6.4% to 74.7%) and Victoria lineage 21.3% (95% CI: -50.0% to 58.9%). VE was highest and protective in 0-5 years old group against any influenza and influenza A and B/Yamagata, but the protective effect was not observed for other age groups and B/Victoria. RDT demonstrated concordant results with RT PCR and virus isolation. Sequencing of hemagglutinin gene showed that all A(H1N1)pdm09 belong to clade 6B including 31 strains (88.6%), which belong to clade 6B.1 possessing S162N mutations that may alter antigenicity and affect VE for A(H1N1)pdm09. CONCLUSIONS: IIV4 influenza vaccine during 2015-2016 was effective against A(H1N1)pdm09 and the two lineages of type B. Younger children was more protected than older children and adults by vaccination.