Effectiveness of inactivated quadrivalent influenza vaccine in the 2015/2016 season as assessed in both a test-negative case-control study design and a traditional case-control study design

Both traditional case-control studies (TCCSs) and test-negative case-control studies (TNCCSs) are commonly used to assess influenza vaccine effectiveness (VE). To compensate for the fact that observational studies are susceptible to bias, we combined both methods to assess VE in one geographical area during the 2015/2016 season, when influenza A (H1N1)pdm was dominant. Our TNCCS covered 331 children aged 6 months to 15 years who visited our hospital with fever, including 182 with influenza, and our TCCS covered 812 pediatric outpatients aged 6 months to 15 years, including 214 with influenza. Influenza infection and vaccination history were reviewed, and VE was calculated as (1 − odds ratio) × 100. In the TNCCS, VE against influenza A was 68% (95% CI 47–81) overall, and 70% (48–83) for those given two doses; against influenza B, VE was 37% (− 12–64) overall and 49% (2–74) for two doses. In the TCCS, VE against influenza A was 44% (15–63) overall and 44% (13–64) for two doses, and VE against influenza B was 24% (− 19–52) overall and 41% (3–64) for two doses. Conclusion: Both studies confirmed significant VE against influenza A, significant two-dose VE against influenza B, and better two-dose VE than one-dose VE.What is Known:• Influenza vaccine effectiveness (VE) varies from year to year.• Observational studies are conventionally used for VE assessment. However, they are inherently susceptible to bias and confounding.What is New:• This is the first report of influenza VE assessment using more than one observational study and performed in a specific area during the same season.• VE estimates obtained in our traditional case-control study were lower than those in our test-negative case-control study, but both studies found significant VE against influenza.