What to know
CDC typically presents flu vaccine effectiveness (VE) as a single point estimate: for example, 60%. This point estimate represents the reduction in risk provided by a flu vaccine.
Point Estimates
CDC vaccine effectiveness studies measure protection against different outcomes. For example, outcomes measured can include laboratory-confirmed flu illness that results in a doctor's visit, hospitalizations, or intensive care unit (ICU) admissions. For these outcomes, a VE point estimate of 60% means that, on average, the flu vaccine reduces a person's risk of that flu-associated 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 bound for this VE estimate (50%) as well as an upper bound (70%). One way to interpret a 95% confidence interval is that if CDC were to repeat this study 100 times, 95 times out of 100 the confidence interval would contain the true VE value. Put another way, there is a 95% chance that the true VE lies within the range described by the confidence interval. This means there is still the possibility that five times out of 100 (a 5% chance) the true VE value could fall outside of the 95% confidence interval.
Confidence
Confidence intervals are important because they provide context for understanding the precision, or exactness, of a VE point estimate. The wider the confidence interval, the less exact the point value estimate of vaccine effectiveness. Take, for example, a VE point estimate of 60%. If the confidence interval of this point estimate is 50%-70%, we can have greater certainty that the true protective effect of the flu vaccine is near 60% than if the confidence interval were between 10% and 90%. Furthermore, if a confidence interval includes zero (for example, -20% to 60%), the point value estimate of VE provided is considered “not statistically significant.” People should be cautious when interpreting VE estimates that are not statistically significant because such results cannot rule out the possibility of zero VE (i.e., no protective benefit). The width of a confidence interval is related in part to the number of participants in the study; studies that provide more precise estimates of VE (i.e., have a narrower confidence interval) typically include a larger number of participants. When vaccine effectiveness is low, a larger sample size is needed to detect a statistically significant estimate.