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.