Influenza vaccine effectiveness questions and answers - CDC

www.cdc.gov

The most relevant sections are these:

**How does CDC present data on flu vaccine effectiveness?**
CDC typically presents vaccine effectiveness (VE) as a single point estimate: for example, 60%. This point estimate represents the reduction in risk provided by the flu vaccine.

CDC vaccine effectiveness studies measure two outcomes: laboratory confirmed flu illness that results in a doctor’s visit or laboratory-confirmed flu that results in hospitalization. For these outcomes, a VE point estimate of 60% means that on average the flu vaccine reduces a person’s risk of a flu outcome by 60%.

In addition to the VE point estimate, CDC also provides a “confidence interval” (CI) for this point estimate, for example, 60% (95% CI: 50%-70%). The confidence interval provides a lower boundary for the VE estimate (e.g., 50%) as well as an upper boundary (e.g., 70%). One way to interpret a 95% confidence interval is that if CDC were to repeat this study 100 times and calculate 100 confidence intervals, 95 times out of 100, the confidence interval would contain the true VE value. A simpler interpretation is that there is a 95% chance that the true VE lies within the confidence interval – therefore, there is still the possibility that five times out of 100 (a 5% chance) the true VE value could fall outside of the 50%-70% confidence interval.

**What do recent vaccine effectiveness studies show?**
Recent studies show flu vaccine can reduce the risk of flu illness by between 40% and 60% among the overall population during seasons when most circulating flu viruses are well matched to the flu vaccine viruses.

This implies that the interpretation above is essentially the way the CDC presents it and the factors that influence effectiveness (based on strains). The percent of nonresponders also likely plays a factor, but it's harder to disentangle those (because some percent of nonresponders may get sick while others don't). Typically effectiveness doesn't take this into account and instead just looks at outcomes of everyone who was vaccinated.