Interesting ways to measure effectiveness. https://www.cdc.gov/flu/vaccines-work/effectivenessqa.htm
How do we measure how well influenza vaccines work?
Two general types of studies are used to determine how well influenza vaccines work: randomized controlled trials and observational studies. These study designs are described below.
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Randomized controlled trials (RCTs)
Observational Studies
How do vaccine effectiveness studies differ from vaccine efficacy studies?
What factors can affect the results of influenza vaccine efficacy and effectiveness studies?
What is the evidence that influenza vaccines work?
How well does the live attenuated influenza vaccine (LAIV) work compared to inactivated influenza vaccine (IIV)?
How does CDC monitor vaccine effectiveness?