Efficacy vs Effectiveness of Vaccines

According to the CDC:

Vaccine efficacy and vaccine effectiveness measure the proportionate reduction in cases among vaccinated persons.

But what’s the difference between vaccine efficacy and effectiveness?

Vaccine efficacy is used when a study is carried out under ideal conditions, for example, during a clinical trial.

Vaccine effectiveness is used when a study is carried out under typical field (that is, less than perfectly controlled) conditions.

Postlicensure studies can often help figure out vaccine effectiveness. For example, the study “Varicella Vaccine Effectiveness in the US Vaccination Program: A Review,” that appeared in The Journal of Infectious Diseases in 2008 “reviewed the results of postlicensure studies of varicella vaccine effectiveness and compared these results with those of prelicensure efficacy trials.”

That study of the chicken pox vaccine found that “the estimates of effectiveness are lower than the prelicensure efficacy,” although several studies found the vaccine “100% effective in preventing combined moderate and severe varicella.”

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