Vaccines: Effectiveness vs. efficacy


During the influenza portion of the Feb. 21, 2018, Centers for Diseases Control and Prevention’s Advisory Committee on Immunization Practices meeting, two pleas from the audience asked the CDC/ACIP to make messages very clear about how protective influenza vaccine really is.

We hear apparently conflicting percentages from Australia, Canada, Europe, and the United States from the many stories/press releases in the news media and from official public health outlets. And the gloomiest ones get the most exposure.1 It can be confusing even for medical care providers who are supposed to advise families on such matters.

A key misunderstanding in many medical and lay news stories is about what vaccine effectiveness and vaccine efficacy really mean. What? Aren’t those the same thing? Nope. They are quite different. And are we sure of what those 95% confidence intervals (CI) mean? Let’s review the “math” so we can explain this to families.

Differences between flu vaccine effectiveness and vaccine efficacy

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